Just to let you know, these fans are not wired. You will have to get a 12v adapter and wire them yourself. Sorry for any confusion this may have caused.
And some PC fans work really well in a terrarium, and others will die quickly due to the humidity.
I was using a cheap PC fan to circulate the air in my orchidarium (a 10 gal. fish tank), and I bought an expensive, nearly-silent fan because I didn't like the hum. Two days and a high-humidity accident later (I left the fogger on overnight), the new fan was dead and I had to go back to the cheap one.