Are you looking for two 120mm fans or one 200mm fan?
If it were my case, i'd go for three red 120mm coolermaster sickleflows. Two for the top of the case and one for a side slot. Make sure you get the 2000rpm version, its very quiet and shifts a good amount of air 69cfm. Here's the link for the coolermaster site where you will find more information about them.
I wont suggest where to buy as that's your choice, however, check out the google shopping search.
In fact you could even get four, and replace the rear exhaust fan so all ya fans match. ( I like things to match, looks ace. )
For ya top fans, make sure they are set to exhaust so the warm air is exhausted out the top and rear of ya case, (there's little arrows on fans, showing the direction of airflow) for the best cooling effect, (warm air rises of course) Ya want the warm air from around the Gpu to be removed as soon as possible.
IF ya need any help with installing them or anything like that, don't hesitate to post.