I learned my lesson at Midnight Rodeo. I went through the whole op with my goggles fogged up and envying those with fans on their goggles.
I looked into what it would take to outfit my goggles with one, and decided to go the do-it-yourself route. I found a 4-6V fan, 1 inch square, on e-bay for $5.00 and bought a single AA battery holder and mini switch at Radio Shack for less than $4.00 total. Through playing around with different voltage/battery combinations I found 2 CR2s (3V each) to be the best, and with a little modification to the battery holder they fit in it well. The fan runs at 60 mA at 5V and the CR2s are typically 900-1100 mAH, which will give you quite a bit of defogging time.
Here are the results. It's inexpensive and easy to do.
http://img242.imageshack.us/img242/5956/img1530dy0.jpg
http://img73.imageshack.us/img73/8490/img1531da5.jpg
http://img80.imageshack.us/img80/9427/img1533di5.jpg

