I've done a few things with spare fans - you can use pretty much any power supply that puts out 9-12V and you'll get good airflow - and yeah - like you and a lot of other posts are thinking, you'll want to make sure it puts out good current.
On my larger boxes where I'm using fans and TEC's, I use an old tower power control - even the relatively dinky ones are 350w and will drive your fans no problem with room to expand. You can get them new for 25 bucks and up. What I like about them is you have a switch on them - gives you control of whether the fans are running or not.