Where can I purchase repair parts for Apple products? I have an old iMac G4 that's well beyond being supported in any capacity by Apple. Two years ago, when taking it apart to upgrade the hard drive and CD drive, I accidentally severed the connector on the logic board for the display. At the time, I looked around to see if I could find somewhere I could purchase a replacement logic board, but pretty much the only place I found were some people selling them on eBay from unknown origins for wildly varying prices. I ended up dropping the matter, as I didn't have a need for the machine anymore, but it got me thinking:

Where can I purchase repair parts for Apple products? Presumably the Apple Certified Repair shops have a supplier - do their parts come directly from Apple? Can a "regular person" like me obtain parts directly from Apple as well? If not, are there any certified repair places that are willing to resell the parts they have access to? Or is eBay, with parts from who-knows-where, the only option?
iFixit has manuals online as well as the parts needed. I've bought and replaced a number of parts from them and their instructions are top notch.

eBay sometimes, but, as you know, that's your own risk.