I'll make it quick and simple. (JK) There is, however alot of different ways to do it. I'll give you a way to get a variable speed out of any regular PC fan. (you may want to go with ADDA brand fans, some of theirs are waterproof, therefor being amazing for high humidity terrariums)
This is the adapter im going to use for my fans inside my terrarium, and will run the ADDA waterproof fans, to keep my front glass fog free.
3-12V Regulated 1000mA AC Adapter - RadioShack.com
This is the ADDA waterproof fans, im going to get the smallest size, because they will be well hidden, and i can set the speed to see the lowest voltage i can use, to keep the glass clear
AQ0624HB-A70GL(T)-LF
(I just noticed that ran off 24 volts, idk what im going to do about that yet. It might run, might not)
Adaptaplug L - RadioShack.com
That thing will plug into the adapter. Since that ones an L you need to find a female L piece to hook up to your fan.
ill finish later, but that will get you started, you can of course get just a 4 pin adapter AC to DC 12v or 24v. Then get a fan with 4 pins also.