Go to the Home Depot website and look up “Gable Vent”. Master Flow makes several models. One is something like 1500 and the other is 1700. They have a thermostat built in but that can be replaced with an on/off switch. If you can’t find it I will find the link.

~NightCat~

Go to your local heating/air conditioning/sheet metal dealer/shop and ask them for a furnace blower from a replaced furnace. It’s almost never the fan that fails in a furnace. I got a 3 speed 2000 cfm unit for free. You can find them smaller. The shops have to pay to have the old units removed and will often let you take what you want. Sometimes they will remove it for you or for twenty bucks or trade. If you explain what you want it they can help with a hood and ducting.

if you can find a fan used or for free, go for it!!! but if you can afford $150 or so, you can buy a new direct drive industrial blower here.

http://www.grainger.com/Grainger/wwg/productIndex.shtml?operator=searchProductBySubCat&segment=HVAC&family=Blowers&subcat=Direct+Drive+Blowers

stay away from blade or turbo fans. while they work great to ventilate your attic, they cannot move air though ventilation ducts all that well. for example, a fan that can move 1000cfm operating out in the open, can move only 100 cfm when trying to blow air into a 8 to 10 inch duct. the only kind of fan to get is a squirrel cage fan as it has enough output pressure to still move a lot of air through ducts.

if all you have for ventilation is an open door and a fan in a window somewhere, then you do not have enough ventilation. you don’t have to spend a lot of money on it. i went the ultra cheap route in my studio. it is a little on the low side but i think it works fine. check it out here

https://www.wetcanvas.com/forums/showthread.php?t=234997&highlight=cheap+ventilation

note that after i posted in the above thread, i added some baffles on the sides and back to extend the hood downward toward the bench top. this improved my ventilation quite a bit. i think i have posted photos somewhere here at WC showing the new configuration.