Where to source replacement internal fans? (For a mid-2011 iMac)
Apple lists service and parts at https://locate.apple.com so you might call and Apple authorized service provider and see if they can get you the part without actually performing the repair.  Craigslist also usually is a good place to buy machines for scrap. They're all sorts of liquid damage and otherwise malfunctioning computers were the chance of obtaining a viable blower are high.  Like anything, you pay for quality inconvenience so shopping for used parts is going to be more of a hassle than paying top dollar to someone who stocks obscure custom oem parts. 

P.S.  Unless Apple lists a pecific part as a do it yourself repair part, you likely won't get them to sell you the part in question.