If you are just running a few fans, then a 12v DC adapter is the way to go. Just be sure that the milliamp rating on the adapter is higher than the rating of all the fans combined, and ensure that you conect them in parallel (ie:- all red wires connected together, and all black wires connected together).
If you want to run more that just a few fans, then i would recommend using the item they were meant for, which is a PC power supply. Most modern P PSUs will only work if there is a motherboard connected, so you will have to short or jump a couple of wires on the motherboard connector. Full details of how to do this are
here.