https://www.gmpartsdirect.com/ and https://www.wholesalegmpartsonline.com/ are amazing resources for finding and buying OEM parts. I've spent countless hours looking through parts diagrams on these sites. If one of them doesn't have a diagram for whatever you're looking for, the other one usually will. These places have saved my ass several times when I was looking for obscure parts for random things. Some stuff is discontinued of course, but you'd be amazed at how many OEM parts GM still sells for these trucks.