The best place to buy both fish and stuff for aquariums is usually a shop called "sklep akwarystyczny" in Polish (aquarium shop), although you can sometimes buy aquarium fish also in bigger zoological shops (sklep zoologiczny). Here's a thread about aquarium shops in Wrocław (it's from 2018 though):
nano-reef.pl/forums/topic/99266-sklepy-akwarystyczne-wroc%C5%82aw/
Or, alternatively, any good and active forum for aquaria, even in Polish so that I can search there?
In Polish it's "forum akwarystyczne". This one seems to be quite active (Warsaw forum):
fawa.pl/
Here you can post/ask about shops from outside of Warsaw:
fawa.pl/viewforum.php?f=32