A couple of years ago I replaced the ones I had with new ones from either Calumet or B&H -- I forget which one. Anyone interested might search for "photographic tray stackers" if "tray ladders" doesn't give a result. My suggestion is to have two. That way you can stack up to six trays in the space of two. If you happen to get the same ones I have the shelves attach with bolts and wing nuts. The wing nuts like to loosen so I suggest a trip to a hardware store and buy some lock washers to prevent that.

http://www.jeffreyglasser.com/