@gggplaya
Be nice...
Op. Fans use very little power, even crappy fans only use about 3w, so you could add 5 fans in your pc and barely make a dent in power.
While not exactly polite about it, gggplaya did make some valid points, how you power the fan depends upon what it needs. Some fans are big 4pin molex, some are small 3 or 4 pin motherboard connectors. There's enough power in a motherboard connector for 2 fans with a splitter.
You can mount a fan anywhere and it's going to help, I've seen plenty of ppl with those micro-towers use zip-ties to hold the fan to the plastic facia inside, just so the fan can suck air through the gaps. There's even exhaust fans that fit in the slots at the back of the pc by the video card, (regardless of what gggplaya believes) but they are loud and not very good quality mostly.