Actually, there are 2 types of power sources for fans. 1 is on your motherboard (these are usually 2 small pins labeled for fans), the other is your regular power supply from your core.
If you'll notice the power wires running to your CD-ROM and harddrive, the have an input, then they go out (like dangling into the middle of space in your case - it is split right where the first plug is).
These are 4 pin IIRC. They look like little plastic rectangles (sorry, I don't know the technical terms, this is not an often discussed area in computers!
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I have two 80x80mm ball bearing case fans (they're attached to each other, stacked) on the back of my case (where the only ventalation holes for extra fans lie).
Your motherboard should have at least 3 separate fan output pins (one for your processor, one for your northbridge MB chip, and one additional one). My Abit MB (based on VIA's 266A) has about 5.