So much cheaper to make it yourself! Plus, you get the added bonus of feeling really proud of yourself for doing it...
Anyways, you probably have several of the parts laying around the garage already. Search the posts, you won't have any trouble finding plans to make one out of one of those styrofoam coolers. I ended up using one of my dad's old fishing coolers and it worked great. I think the only things I had to buy was the thermostat, the hydrometer, and a lamp kit.
I used a small hole saw (about the diameter of a wine cork) and drilled some holes around the top, then plugged them with wine corks, so they were there if I needed added ventilation. I wired up a lightbulb to the thermostat. I found some wire mesh in the garage and bent it so that it would span the cooler and sit approx 2" from the bottom, and I put an old cake pan full of water under that. When the water got low, I could refil the pan by pouring water through the mesh. Keep an eye one the hydrometer, whenever I needed additional humidity (ie, the last three days of incubation), I added wet towels lining the sides of the cooler.
Good luck!