I picked up a double window fan at Lowes for this exact purpose. I was $39 i believe. You can look at it on their website. It works awesome. The great part is, you can take it out of the window when not in use.
If the space is small, and the budget tight, you cant beat a window fan. If you want a little more firepower, you can go with a box fan.
Dont forget that you need a air source comming in if you want any air to go out.