OK just to clarify, top corner and not your powersupply fans? How many case fan mounts do you have? They are the square things with holes/grills in the middle.
You should have a mount for a front case fan near the bottom in the front. I would install one there, blowing in. They arent expensive, but hte more you spend, generally the quieter they are. Pabst makes good quiet fans but are a bit pricey. You are going to want a 80mmfan. Read your mobo manual to find out where to hook up the power and probably a fan monitor.
Try newegg or
http://www.plycon.com
You can even get led fans for $10 and cold cathode for less than $15.