CS,

You need the fans on all the time so batteries aren't a good choice.

The adapter will cost you like 9 bucks unless you have one sitting around collecting dust. And it would have to match the rating on your pc fan.

Mine's 12 volt but some may be 9v. I'm not Joe electronics but those are common voltages for adapters. Should say on the fan what it is rated for.

My adapter came with a little white socket. Turns out I did not snip wires on the adapter but instead bought the compatible plug and wired that onto the wires from my fan(s).

Only thing is to test the polarity of the wires. The fan will only work if you get that right because the plug and socket only fit one way. You won't hurt anything if you get it reversed while testing for that. Just don't be holding bare wires obviously - or - well - considering you might be completely zonked, take my word for it.

Anyway point is you might as well put the adapter money toward a good fan. And the one below is about the best you can find under $20. Walmart has 'em.