I have a 1999 base model Blazer and I'm looking for dash parts like instrument panel covers, and clips for the instrument bezel.
I have already replaced motor and pretty much everything under the hood and front end, I am working on the interior now.

 

You can try LMC truck, https://www.lmctruck.com/ and perhaps Rock Auto. But for non-mechanical parts, a wrecking yard might be your best bet. This is where I got a really nice cargo area carpet for my 99 4-door. I was able to use the VIN to confirm the interior color of the ones in the U-Pull yard so I only had to search those.