Don't even bother with the 12 volt, besides, it's DC volts and you need AC volts. You have to get the 120 if you want some good flow. A couple of suggestions for you:
The ones Radio Shack sells produce the proper CFM (cubic foot per minute) and I can't remember what that is, but the box will say. Reason I'm mentioning it is that I haven't had good success with the Radio Shack fans, so I would suggest anything else that runs at the same CFM. Other folks have told me that they had good luck with RS fans. Your call. Radio Shack also sells the fan covers to make a nice trim on the canopy. They're just a few bucks each and they will work with any brand fan usually.
Next, be sure to buy the right sized fan. I have a pretty big canopy and a four inch fan is just too big to fit. Drilling a hole for the air would have only left like a half inch of wood on either side. That was too scary for me, so I went with the three inch fans. You can buy a hole saw that fits a standard drill at just about any hardware store.
The power is something you want to be real careful about. One side of your ballast will be producing somewhere over 1000 volts. DO NOT CONNECT TO THIS SIDE OF THE BALLAST. Actually I would recomend not connecting to the ballast at all. I realize that you are trying to get the fans to go on and off with the lights, but depending on where in the country you live, you may want the fans to go off before or after the lights do. Certain times of the year I run fans 24/7, but I live in the desert. Just buy another timer and you can pick up a cord for the fans at the hardware store. They sell a cord that on one end has bare wires ready to connect to something that operates at 120 volts. The other end plugs into the wall.
Hope this helps
!!!