Nowhere near as many ballet memorabilia sellers as there used to be (in fact I only ever knew the private dealers, and then of course there was the Ballet Shop when it was there. There are some music related ones like Schubertiade that do it; they are in Massachusetts I think. Mostly if they sell/sold arts related/theater/opera, they sold ballet also. And then of course the larger houses like Christie's, Sotheby's, Doyle, etc., but a collection would have to be historically fairly impressive for them to take it on. There are still a couple of private sellers on eBay who deal in Elssler/Taglioni etc. era things and more. They're pretty easy to find.