Ebay rocks for cat trees.
They have really awesome cat trees for ALOT cheaper than in stores, but the only thing is, you have to pay a chunk for shipping costs. It's not as much as you would pay for a cat tree in stores, but it is expensive. But they have great cat trees there.
Also, petguys.com has some stuff there that I have found to be the cheapest out of most online pet supplies. Otherwise, I'd just go with petco.com.
Here's a link I found on ebay to kind of give you a hint of what kind of things they have for cheap.http://cgi.ebay.com/78-Cat-Tree-Hous...QQcmdZViewItem
here's a link to a pretty good litter box for pretty good price at petcohttp://www.petco.com/Shop/Product.as...23+107+30+5658
Otherwise, Walmart has everything, for pretty cheap. They have litter boxes, and toys, and food bowls, and treats, and grooming stuff. All for a couple bucks each.