I haven't been to Białystok myself, but I can't see any reason why it would be different from the rest of Poland in this respect. Large hypermarkets like Tesco, Carrefour or Auchan have it, harder to find in smaller stores.
Pepco shops are very cheap and have good design.
Thanks to both of you!
JonDoe, ya I figured If I didn't get any replies here I would post in general Poland form, but ya you're probably right. I will keep to the big shops. I think there is a Tesco nearby, - Traveller1687, Good to know, I will check if there are any Pepco shops there. Thanks again. Dziekuje