There's a guy on ebay that sells cooling fans. I think his store name is robobugs. He claims that his fans are very tough. I got the 12v version and am planning to run it directly from my 4S lipo pack. Less work for the bec and when you run a 12v fan on 4S lipo, it's like a little hurricane under the body. I'm running a 12v 40mm fan on my motor and haven't had any problems.

Just keep in mind that a fan isn't a good fix for an overgearing problem. I like to stay conservative with my setups. Well, conservative in the sense that I like to set things up so that they work without always being pushed to the limit. I set my cars up so that they do not thermal under normal driving conditions, then add a fan to keep it in the comfort zone for when I push it harder than I usually do. If your MM is thermaling and you aren't doing any really hard driving, I'd look to the gearing or even the motor, esc, battery combo.

What are you running anyway?