The Travelling Shop, or T-Shop, is a magical store that moves from place to place. It has stops on every continent, most cities, and even a few islands. As one may imagine, this feature makes it very hard to find- so much so that the largest Club on the Discworld, the T-Shop Spotters, was formed for the purpose of keeping track of it.
The T-Shop sells some of the rarest, hardest-to-find items on the Disc (at bargain prices). A full list of them may be found here.