Where to source replacement internal fans? (For a mid-2011 iMac) I'm looking to replace one of the internal fans in my iMac (mid-2011, 21.5" version) - is there anywhere these can be sourced?. 

I can't seem to find it online, and only see the part mentioned in passing in some of the iFixit disassembly instructions.

The reason for the replacement is that the fan is noisy, and I wasn't able to remedy it after clearing out the dust with plenty of compressed air.

This is the fan I'm looking to replace:
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.