Cut the plugs off the fans. You will use only the red and black wires, ignore the yellow or white wire as it will not be needed.
I use variable voltage "wall wart" type plug in power supplies that have 3v, 4.5v, 6v, 7.5v, 9v and 12v settings so you can vary the fan speed according to your coooling needs. You will also cut the DC plug, not the 120v wall lug, off the power supply then attach the fan wires to those two wires using crimp lugs, wire nuts or similar.
I had a big write up on exactly how I did mine on AZReefs.com but the site recently folded up so lost all my threads on builds. I may have it somewhere else and will look.
You can find the power supplies at WalMart in the camera/electronics dept. They are used as universal power supplies for cameras, cell phones and similar DC products. Should be $10-$15 at most, I paid $4 for mine at Harbor Freight Tools but I don't see them there anymore.