There's a Goodings near Downtown Disney. I think that would be the cheapest of the close places. You would need to call a cab I believe. I've never been there, and it may be possible to walk from the DD bus stop, but even if it is walking distance, you won't want to carry your purchases for any great length.
As long as you're calling a cab, you may just want to go to Publix near the entrance. You would save a little money. If you don't want to call a cab, you can try the Gourmet Pantry at DD or your resort sundry shop.