I'd suggest www.wine-searcher.com

It won't get you other collections, but it will find specific wines that are being offered on consignment through brokerages. Including mine, which are being offered through OWCwines.com who is my broker.

You can also buy wines at auction through such auction houses as Sotheby's, or Bonham and Butterfields. The wine-searcher site will allow you to buy specific wines that are being offered for private sale, while the auction house will allow you to buy assorted lots and collections in whatever groupings the auction houses chooses to group them together.