Finding parts has been the problem with Apple from the start. They want control. They want a monopoly. They want to force you to go to them for repairs.
They want to put me out of business.
Finding a reliable parts supplier is essential to any business. Now what is important in a supplier to me may not be to you.
Here's what I personally look for: I only want to do a repair once. My time is where I make make a living. I don't want to test your parts, that's the suppliers job. I need my parts in a timely manner. I need the correct part, I want a supplier that is good enough to know there are subtile differences in different models.. I want a clean part. I want my suppliers to speak English as a first language. I want a supplier that tells me how to install the part down to the screw size and what to watch out for. The better warranty I can get the better I serve my customers. So I know to get what I want I'm going to have to pay a premium and I don't mind that, you have to make a living too. I appreciate a supplier that gives me tier level pricing so that the more I spend with you the deeper the professional discount. I want a supplier for the rest of my lifetime. I do not want to have to spend and hour trying to save 10%.
So I have moved as much of my parts buying to iFixit as I can as they address all the above issues. I'm not just a customer to them, I feel like we have a partnership. win-win
As the icing on the cake they have customer support people like @kaykay and @kelsea. It just doesn't get much better than that.