Newegg.com

They have cheap PC fans, way cheaper than any store selling PC parts that I've ever seen (from radioshack to bestbuy to PC specialist stores). You can narrow your selection down by size, price, cfm, brand, and other specs too. Also great because it has user reviews. That links to a rosewill 120mm fan.

Only thing that might throw you off is the shipping, but it's still cheaper than any store I've seen and if you buy two or more fans it gets cheaper still.

Hope this helps some of you out. Also, if you ever want to buy computer parts, go here, it's worth it. Even if you don't plan on buying it there, the user reviews are gold.