You would have to modify your case since there are no openings for an extra fan. Buy a 90mm or 120mm fan, make a template, cut out the side panel. I'm not if your motherboard has an extra fan plug, you can get a Y cable for 2 fans on the same plug.
Depending on the case you might be able to get one of those cheap pcie fans to help with some circulation... i would reccomend against this if its blowing in a direction where you dont have exaust though.... If you don't want to cut into your case like those guys are are suggesting i would suggest rather than adding fans, you replace the ones you have with better fans of the same size. Noctua makes a fine fan.