Ballgowns molt. Latin dresses shed. The hand-sewn fringe snaps off, and the hand-glued rhinestones peel from their backings. Feathers are the worst; I heard a DSI commentator say that the dance floor looked like someone had shot a crow after a black feather dress had left the floor.
I find it tragic, having spent hours and hours stoning dresses. At risk of convincing you that I am a complete weirdo, I will admit that I often pick up stones I find on the floor of the dressing room at competitions. It seems so wrong to just leave those expensive rocks lying there.
Dreamtime Creations while living in the US and Modastrass while living in Europe and can recommend both of them. Dreamtime Creations has the better selection of Swarovski colors, though. For larger quantities, Chrisanne has competitive prices for Swarovski, Preciosa, and their own in-house line; Chrisanne also sells their own rhinestone glue in both large and travel-sized bottles. DSI does as well. Other glues often used for rhinestones are Gem-Tac and E6000.