An electronics supply store. I don't know any really good ones though, but if nothing else, you could try "The Source by CC" (formerly Radio Shack.. I wonder how Monza feels that they renamed their store and copied his name..).
Another place that has a good supply of fans (some may be surplus though) is Princess Auto. I got a pile of fans from there, ranging in size from like 1" up to the usual 4"-5" ones and so on. Cheap too, usually.
