You could build a little regulator that reduces the voltage down to 12v or you could use a ubec to get the voltage down to 6v then use a 5v fans. I think there are some threads around here that may explain how to build a regulator.
Brushless Jammin Truggy on NEU 1515 2.5 D on Monster mamba max with maxamps 5000mah 6s Lipo
Gorillamaxx Revo CF version on Mamba maxx ESC with NEU 1515 1Y on Maxamps 4s 8000mah Lipo