Where can I get Buddha's teachings of the Dharma practice?

You probably know this already but the name of the texts that preserve the teachings of the Buddha are called suttas (or sutras). Generally speaking, they are the closest records of what the buddha might have taught.

Other than these texts, the teachings of the Buddha survive through transmission from master to disciples across millennia spread through many branches.

These are likely the only sources of the teachings and both present problems of trust and authenticity (This does not mean current teachers are not to be trusted, nor that texts are not to be trusted, only that one needs to be at least aware of the problems one may encounter).

On the suttas, they are voluminous and were written down at different periods -- some of them arguably composed at different periods as well. One particular collection is often considered to be the earliest part (and, to many, the most authentic): these are the four nikayas/agamas (Dīgha, Majjhima, Samyutta, Anguttara) plus the Vinaya. Translations of these collections to many languages are available online. They are also available in physical books in english (see here, here, here and here).

These collections had translations preserved by many schools across many countries through centuries without much exchanges with each other, in particular, Tibet, Sri Lanka and China. The oldest texts are preserved in Sanskrit, Pali, and Classical Chinese, respectively. None of these are believed to be the language the Buddha spoke (some speculate that Pali might be a language close to their tongue).

A fifth book, Khuddaka Nikaya, is a miscellaneous collection and seem to have more divergences across recensions, mixing what seems to be older and newer compositions. Some schools also developed a third basket: the Abbidhamma, whose origin is not clear (some believe it was a project initiated by the Buddha to systematize the teachings, some believe it was developed by the schools as they attempted to solve conflicts between them or to clarify ambiguous passages, etc).

Two important take aways from the four Nikayas:

• all schools recognize these collections as preserving the Buddha's teachings;
• comparative analysis of the existing recensions show a remarkable consistency in content for texts of these collections.

Naturally, the buddhist schools grew apart and dispersed geographically. In doing so, they developed their own literature on top of what they already had: meditation manuals, philosophical treatises, commentaries, but also, some of them developed new sutras (that is, texts narrating the Buddha's dialogues and teachings). These new sutras have a distinct style and emphasis, and are usually recognized as Mahayana sutras. Some regard them as superior in their teachings than early texts, some regard them as equivalent in meaning to the content of the nikayas and some do not regard them as authentic (the fact that they were composed much later is mostly accepted by scholars -- some buddhists have another explanations for their origin). Exemples of Mahayana sutras are Lotus, Diamond and Prajna Paramita.

Personally, as someone who studies alone, I find the nikayas to be the best bet to get to the closest of what might have been the Buddha's words -- my own opinion, of course. The closest, not the actual words verbatim, because it seems he never wrote from his own wrist, and you know, oral transmissions, translations and editions have a way of introducing errors.

On that note, two scholar monks, Bhikkhu Sujato and Bhikkhu Brahmali have done an interesting work defending the authenticity of the nikayas: The Authenticity of the Early Buddhist Texts (pdf here).

Also, the Theravada, considered the oldest surviving branch of buddhism is the most known tradition to preserve, study and practice according to the nikayas & vinaya. Moreover, scholars of early buddhism (in contrast to late/Mahayana buddhism) focus on the nikayas. In contrast, many Mahayana schools, although recognizing the nikayas as authentic, generally are not known for studying them.

Finally, for the most part, the reading of the nikayas is not really obscure (as one might expect after reading texts from other religions); I think they are very readable. Of course, when they get deep they are remarkably deep. And sometimes confusing -- after all it's arguably a 2500 year old text. However, there's plenty of books (commentaries, manuals, etc) that help clarify specific sutras or concepts. And for everything else, there are forums and Q&A like this to help.

The nowadays Vipassana (Mahasi Saydaw & his many direct or indirect descendants) reference Satipatthana Sutta. However, quoting an excellent Theravada reinvents vipassana blogpost:

If you read the Satipatthana Sutta, the most-used one, and if you know how to meditate, you can say “yeah, parts of that are a pretty good description of what we do.” (Other parts are nothing like what people do now. I think that’s important, as I’ll explain in a later post.) If you had no idea what meditation was, the Sutta would not seem like much of a guide.

Your prospect of getting straightforwardly useful meditation instruction from the text itself is bleak, unless you want to repeat what people like Mahasi Sayadaw did (and they spent lifetime studying and refining the technique). (I am sorry you had bad luck with your previous teachers.)

It is even more the case for devising progress metrics. Theoretically, there are 3 trainings in buddhism (sila, samadhi, panna) which are to a significant extent orthogonal. There are some traditional metrics (8 jhanas (pre-buddhist AFAIK), 16 insight stages (later commentarial), 4 paths (Buddha original)) but there is often little agreement on what they actually are, including among people who devoted significant time to studying them (see e.g. Shankman's The experience of Samadhi; or online discussions in communities like dharmaoverground.org; see also what unnamed Pa Auk monks say about Mahasi's sotapatti certificates -- there is a great deal of rivalrly and "we are right, you're wrong" attitude) let alone some codified assessment procedures. You've seen some of that, I guess.

"Progress" is not a monotonic (not to say constant) function, more happy now does not necessarily mean better off long-term. The goal of Buddhist practice (as I understand it) is freedom from suffering, some say "freedom in the midst of suffering" -- implying it is not absence of suffering but rather a different way of relating to it.

One of the universal characteristics (3C is Buddha), anatta, means uncontrollability, and it is also a characteristics of your practice. If your task is to have your practice under control, odds are that you will suffer because the practice simply does not care about what you want :)

Do a long Vipassana retreat; walk the path from where you are right now, before you dream about what your ideal path is.

(PS If you want to read original texts, you have to get over words like "enlightenment", they are really there.)

...without things that I don't need, especially without the idea that the purpose of Buddhist practice is to achieve enlightenment...

If we walk the path to the higher goal, the lower goals will be fulfilled along the way.

...the reason for me to practice is to reduce my suffering in this life...

Sooner or later, we'll realize that the only way to reduce our suffering in this life, is to help reduce the suffering of others. To have the strength to do it, we'll need very strong motivations.

For example, the happier I am, the harder and more effectively I work...

There is nothing wrong in using the teachings to achieve mundane goals. However, this achievements are only temporary, and if we are not able to see this, they will only lead to more suffering.

Our ultimate purpose is using the practices to liberate ourselves of all relative conditionings, not only to improve them.

Tenzing Wangyal Rinpoche