Ashitaya is a small health store, but you will find everything you need (including local organic vegetables) that you don't find in a regular supermarked. I go to Ashitaya to buy my hempseeds and TVP meat. They even have some types of european vegan spreads that are really good.
The shop is located in Makomanai, a 20th minute subway train ride from Sapporo station. It's worth the trip. The only negative thing about Ashitaya is that they sometimes use a long time to restock an item. If you see something you think you might need, I would recommend to buy it then and there.
Cons: Not super sentral, Need to restock faster