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.