Here's the method I usually use for buying new GM parts.
1.) Check an online GM dealer parts website like
https://www.wholesalegmpartsonline.com/ to find the GM part number of what you're looking for. If the part you're looking for isn't in those diagrams, it's probably discontinued. Unfortunately these sites tend to just remove parts from their listings entirely when they're discontinued. You can still find those discontinued part numbers on
https://nemigaparts.com/cat_spares/epc/chevrolet/ but the site is a little clunky to navigate.
2.) Once you have the GM part number of what you're looking for, throw that into all the popular vendors search engines. Amazon, RockAuto, Summit Racing, JEGS, etc. Chances are one of them will have it cheaper than the others. In my experience Amazon or RockAuto usually win.
3.) If the GM part you're looking for is discontinued and you can't find it for sale new at any of the vendors I mentioned above... Check eBay first and foremost. You'll be surprised at the weird NOS parts that people will list there.
4.) If that fails, aftermarket suppliers like
https://www.lmctruck.com/ and
https://www.classicindustries.com/ sometimes reproduce factory parts that have become hard to find. Same goes for
https://www.holley.com/, they're reproducing lots of discontinued parts for these trucks including the factory style floor mats now.
5.) If you've exhausted all other possibilities, check
https://www.car-part.com/ and
https://row52.com/. These sites index salvage yard inventories across the country, so you can look up suitable parts trucks in yards near you. Car-Part is for full service yards, Row52 is you-pull-it yards.
Hope that helps.