A friend of mine has a couple of fans built into holes in his bedroom walls which he leaves running all day and says that they negate the need to run the AC.

I was thinking of trying something similar, but finding the right fan might be a problem, as I wanted to try mounting one on my security bars, so it would have to be thin like a car fan. If I bought a car fan, would it be easy to find a 12v transformer. I assume it would as they are used for low voltage lighting aren't they?