You don't really need an extra fan, it works fine with one.
Any 120mm fan will fit, but not all are designed for high pressure applications like a CPU cooler.
You need to choose a fan with high static pressure.
Also, the current fan is PWM controlled to slow down when the CPU is not hot.
You want either match this fan with a similar one from Coolermaster, or just get two constant speed low speed fans for both mountings.
This fan is very good for silent CPU coolers:
http://www.nxstek.com/NXS-nexus120mmrealsilentcasefan.htm