Where can I purchase repair parts for Apple products?

Apple resellers are allowed to both perform repairs where the broken part goes back to Apple and just sell you a part. I believe their costs are way higher for the costs of an "one way part" so in practice, this does not happen much on major components that are not commodity parts.

In your particular instance, I would look to see when that specific model goes vintage. Presumably, Apple would then sell their stock of parts to the repair centers and for a limited time, there might be a nice supply (prices low) and no risk to the people carrying the part that their costs to carry inventory will raise the price of that part.

Even from reputable sellers, many parts that exist on the open market are scavenged from machines that are damaged (sometimes by liquid) and disassembled for parts and carry only a short warranty if any from the seller.

This is a good thing in almost all cases - reusing perfectly fine parts when they are properly tested is inexpensive and proper. The flip side, is what machines get damaged and don't get fixed? The worst kind of damage.

An iMac is very much not at risk from liquid but other things like power surge, can affect parts in an invisible way. Again, the question is of self-selecting. Which macs don't get cheap parts to fix the issues they have? The ones with major problems to either the display or the logic board.

For these reasons, I prefer to buy parts from a company I trust to know why a used mac isn't getting fixed so I can get good parts from them. Similarly, I'll gladly buy the damaged mac from the person that wants to sell it for parts so I can evaluate the damage myself. They are usually forthcoming why and how it got damaged ( and also pricing my offer knowing it may be a total waste.)

As always, finding a good mechanic - someone who knows the business and has experience with hundreds of repairs on your specific model and thousands of repairs under the belt is more important to knowing when parts can be used.

The sad fact is specialized and highly reliable parts outside of Apple's supply chain are too expensive to be widely available. It's hard for consumers to know if a good price is a lucky break or the seller has more information of the part's heritage and passing on hidden risk to the buyer.

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.