I bought 2 of my 3 cat trees on Ebay. If you go under the pet section and look for cat condo's or cat furniture you will see quite a few different ones. They come in all shapes, sizes, and colors.
Some of the listings on Ebay are auctions and others are on a buy it now basis.

Here are a few pictures of what I have. The first one I bought outright for $100.00 and the second one I won the bid on for $129.00.