I would recommend staying in the Hirafu area... that is where most of the restaurants are. If you use Hirafu as your base of operations, you will be able to access the other resorts (Grand Hirafu, Niseko Village, Annupuri, Hanazono) either by crossing the peak (all the resorts are joined at the peak) or by shuttle bus which is free if you have a "All Mountain" lift pass.
The bus from the airport to Niseko will cost you 2300 Yen each way per person.
As for supermarkets, there are 2 convenience stores and a few pricey deli's in Hirafu. If you want a proper supermarket, you'll have to take a 15 min taxi or bus to the adjacent town, Kutchan.
Get ready to have the best ramen you've had in your life