I use the 4" fan from Radio Shack. Hooks up to AC without having to convert it. I like that fan because of the metal housing and it is easy to hook up to some L brackets to mount it. I have mine pointed at the bottom of the incubator so it just rotates the air across the bottom section of the heat tape where I have a couple of water jugs to help stabilize the temps. No problems with the set up at all so far.

Radio Shack Fan