I have photos and a description of my spraybooth on my blog:
http://billweckel.com/2009/07/diy-airbrushing-spraybooth/
Squirrel cage fans are pretty easy to find on eBay, with and without motors. Search under carpet cleaning equipment. That's where I got mine. After having built one, I don't think I'll use a squirrel cage next time. IMHO, and from my experience, it's overkill as far as safety is concerned. Unless you thin your paints with gasoline, I wouldn't spend the extra money on a squirrel cage.