There is a fairly small but well-stocked camp store at Snug Harbor and an even smaller store at Windigo, both near their respective NPS information offices. You shouldn't rely on either of them for equipping or provisioning your trip due to the high prices if nothing else, but they provide a handy safety net if you discover you've left something behind, and they're popular with those just coming off the trail looking for food that doesn't require the addition of hot water. They sell dramamine by the dose, for if the ferry ride home looks like it's going to be rough.
If you want a souvenir from the island, there's an assortment of t-shirts and sweatshirts in the camp stores, and Rock Harbor Lodge which operates the stores has a small gift shop sharing space with the Greenstone Grill with a larger selection of merchandise, including knick-knacks, plush animals, and the like... so don't go and help yourself to things you find in the wilderness. Removing samples of greenstone from the park is prohibited, and tampering with the wildlife and environment in general is discouraged. "Take nothing but pictures and leave nothing but footprints."
