There are a ton of places to get fans online. I got a great deal last year on a bunch of 4" 105 CFM fans on EBAY.
What are you trying to do with the fans? Cool your lights? Cool your tank by evaporation?
If you have halides I would suggest enclosing them some how and create like a wind tunnel with a fan on either end, one exhaust and one intake. This works well in dual halide setups although not necessary for single halide setups.
If you are trying to cool your tank by evaporation and you have a sump then aim a fan at the water's surface. This should help a lot. Plus the higher the evaporation rate the more kalk you can drip.
If you are looking to purchase some fans then I would suggest EBAY for starters or maybe
Here or
here or any electrical retailer should carry some.
If you are going to place the fan in an area where it could get wet, or possibly dropped into the tank then you should consider 12 volt DC for safety reasons.
Hope this helps.