Varney's Book Store is the definitive K-State store. They feature an almost comprehensive merchandise collection and--thanks to a partnership with the university--even run a store on campus in the Student Union.
The vast majority of text-books are available here, along with all of the art supplies, office supplies, clothing, gifts, and all that would be expected from the "official" K-State store. The close university tie is certainly used to it's fullest, as you'll find that almost all items are the full retail price associated with "official" products.
For convenience, you can't beat a trip to Varney's: they simply have everything K-State. But if you're looking for savings or something a little more personal, you'll be better off searching around for a store a little off the beaten path. Or at least off campus.