There is a good market at Colter Bay Village at the Tetons, and West Yellowstone has two nice markets in town just outside the Yellowstone West Entrance, and Gardiner has a good market as well just outside the Park's North entrance. Inside the park, you'll find General Stores with reasonable supplies at Canyon, Mammoth and Fishing Bridge. We always want to grab fresh produce and dairy every few days and these are all convenient places to pick them up and keep us from trying to stock perishables for a full 14 days
We usually freeze items such as beef, chicken & fish for grilling - and we love the instant mashed potatoes, 90 second rice and pasta mixes which round out meals nicely and are easy.