What fans are some racers using for cooling the ESC for mod in 1/12 scale?

Whatever you can get to fit under the body, usually a 25mm or maybe 30mm.
Usually on their side pointing at the ESC as there's seldom room on top of the ESC.

Any specific brands, or are they all about the same?

Are you sure you need one? Generally, geared right, you shouldn't need a fan to run 17.5 or mod - for your motor or your speedo... Are other people running an ESC fan where you race? I would just run what someone you race with is running (or something similar anyway) as I've never seen a fan on a 1/12th scale when I run it... Good luck with it.

You shouldn't really need a cooling fan unless you terribly over-gear your car. BUT, that's exactly what racers do, over-gear the poo out of their cars in the quest to go faster and win. Crazy, isn't it?

Correct for 17.5, but you almost certainly need one for open Mod.

Never even seen a fan for the motor in 1/12

OK, thanks for the input. I think that i found my answer.

And with that post we can lock is topic