I bought a 6" fan at Princess Auto. It is an ADDA AA1751MB-AW fan, ADDA is the manufacturer. It apparantly flows about 190cfm.
I believe there is a Princess Auto in Newmarket and Mississauga www.princessauto.com
I have as of yet not blown myself up. I use a lot of enamel paints, I have blown lacquer thinner straight up the pipe through the fan and not had a fire.
I currently have it setup like this.
I cut a piece of plywood the size of the window near where I airbrush, I can slide the window open, then slip the wood into the window track, I cut a hole in the piece of plywood the size of the fan blades. I attached the fan to the wood with screws, made a duct that mated the fan to the 6" dryer vent tube, then got a good sized cardboard box. I cut a hole in the side of the box toward the back and up. Taped it all up nice with duct tape, attached some lights inside and away I go.
One thing, the fan looks like a very large and very powerfull computer case fan. I bought it with no plug on it, just a couple of wires coming out the side. I wired it up to a 2 prong plug and it works like a charm. Princess Auto has all kinds of stuff like this, fans and whatnot, not to mention they have a ton of other tools that are usefull for modelling. Prices are rediculously cheap. I bought a set of micro drills for 3.99. a pack of 20 or so.