Generally street sales are rare. There are basically no open air markets anymore. You usually habe to know someone. I would recommend asking some uni students who will hook you up if you give them cash or share. Gypsies and soccer hools also sell bud but you have to be careful so you're not ripped off. Never front nor pay before inspecting.
Good luck and happy toking. Fyi most the bud in Poland is high quality Dutch type bud. Sometimes There is afghani hash.