Stodola is still there (Sunday 'till 14:00), and I'm not sure what it was like last time you were there, but prices are quite high. You should be able to get what you need there tho'. There's also a photo store near Rondo ONZ called Czarno Biale http://www.czarno-biale.pl/sklep/ that has a lot of chemicals and film, probably the best I found. They should do scaning too, other than that there's the chain ProfiLab which I never had good relations with, but you might get better results, there's one behind the Empik on Marszalkowska.
The bad news is the water is terrible, very hard and spotty, full of heay metals too - you can almost taste it.
Hope that helps.