I was at IKEA of all places Saturday and they had a unique one that actually looked like a tree! It had shelves and the top had a shelve with the tree branches sticking out of it...$149...
Other than that, you might try Cat Fanciers or whatnot...they should have a website or phone number and usually have links to people who build them...I was talking to one at a show once, but ended up buying one from Wal-Mart and modifying it a bit...it just worked out the Wal-Mart one had the same carpet I had laying around so I could add to it and it only cost $30
Another thought might be the Bargain Finder...you know, that place an ad for free paper that comes out every Thursday and sold at 7-11 etc...not sure if they have them out east?