There are few or none stationary growshops in Poland. It's probably because of drug related paranoia due to Poland having so conservative catholic society (or everybody believing it has). Personally I find it better that way, cause it such shops could be a step to delegalizing seeds (like in Germany or USA) and that would be a disaster.
If you plan to stay in Poland for a while, better check out some online shops or use allegro.pl (it's like ebay or smth, you can buy there many things, but not the seeds). If you want to buy seeds try to google sensimilla or quakemind.