Upvote:0
It may not be a perfect answer as I cannot figure out how to get SIM card before (You can try to do it somwhere near the border), but here's an offer of one of most popular networks in Poland - PLAY:
http://www.play.pl/en/
They seem to have a nice offer of unlimited internet access.
You can try other networks: Orange, T-Mobile or PLUS as well. I believe You can buy their starter packs (SIM cards) in almost every shop.
Upvote:2
If you intend primarily to use you smartphone to chat, you could take a look at Chatsim. According to it's website:
ChatSim connects all your ChatApps around the world. This way you can chat free of charge and without any limits with WhatsApp, Telegram, Facebook Messenger, WeChat, Viber, LINE, QQ and all other instant messaging Apps. With only β¬ 10 you have unlimited words and emoji for a year.
They also mention that they will "deliver it wherever you are for free.". I suppose this includes Poland.
Chatsim also has voice plans, but those are out of the scope of your question.
Disclosure: I am not in any way affiliated with Chatsim. I never even used it, just read about it in a magazine and it seemed like a good choice.
Upvote:2
You could give GO-SIM a try, they have a European SIM card. As with other multi-country operators, it's call-based rather than data based, but it will save you the time and effort of buying a new SIM in each country you visit. They also have a Data SIM, but this may not work in every country in Europe.