is it easier just to buy in the airport shop ?
By them at the airport shop, there's nowhere else (apart from a petrol station) within a 15-minute walk of the airport. There is a supermarket in Modlin but that's going to be a good half an hour (and the road to town lacks any kind of pavement and so pedestrians are forbidden from using it.
I've personally sent people buying 20+ cartons at a time (cartons, not packs).
chill with a few vodkas
If you want vodka, Lublin's a better bet, Warsaw is more about beer now.