Upvote:2
For Russia: an invitation is mandatory, but it is given to you by the hotel or the travel agency you book the travel, if you are staying for tourism. It is very easy to get, and as far I know, it is not really checked after you get it.
[In my case, I had to change completely the plans, and cancel a tour, but I did a much shorter tourist travel in Russia privately organized. Russian travel agency and in any case Russia immigration were ok].
Note: if you plan to stay by a private accommodation (e.g. a friend), this is more complex, and your friend needs some official documents. Embassies website have the links.
On the other hand, you are a Nigerian. Many Nigerians are not following the rules, so embassies and immigration officers are much more strict.