It depends how you want to do it. You could just use your main batteries to power a standard 12V fan, or you could get a 5V fan that can plug into your receiver, like this one:
http://www.allelectronics.com/cgi-bi...164&type=store
You'd need to splice the wires of this with the wires of a receiver connector, though.
SH Z-Car, Custom Crawler, 8s Savage,
12s XTM XLB 1/7 buggy,
4wd 4-link rear/IFS Pro4 truck,
Custom Hyper 10 Short Course,
Belt-Drive Mammoth ST 1/8 truggy, 4s 17.5 MM Pro HPI Blitz