I used a lot of the advice from Charley's greenhouse when I built my GH.
I've provided a link to their tips page. I used their formulas to
calculate how large a swamp cooler (evaporative cooler) I'd need as well
as how large a heater. My GH was 8x16 and I put Charley's swamp cooler
in it which was over kill, but when I expanded the GH to 8 x 30 I didn't
have to get a larger cooler. It still works fine.

http://www.charleysgreenhouse.com/index.cfm?page=_tips

you can find swamp coolers at Home depot or Lowes or google them online
once you've seen what they look like at Charley's. I just went with
Charley's because I knew I could get one there and didn't want to
search. My friend got one at Home depot 3-4 weeks later for half the
price. You will have to develop your shopping skills. If they have
Graingers in your area they are a resource for thermostats etc, too.
Usually at a cheaper price. graingers.com (I think)