So now that I've endured all that goes along with the leaking drain hoses from the sunroof of my H3..., Water into the cabin from under the glove box and onto my lap from the overhead OnStar speaker, I need to replace the carpet and padding. Any recommendations on how/where should I start that process?

 

Fix the leak first.
Check ALL of the drain tubes for connection to the sunroof pan. Use RTV and zip ties to keep them securely on the nipples of the sunroof pan. Cut the ends off of the rubber pieces the tubes attach to on either side of the rig.

There are several very helpful posts here with photos and "how To" instructions.

Google is also your friend:
Photos: https://www.google.com/search?q=humm...=1920&bih=1069
Videos: https://www.google.com/search?q=humm...bih=1069&dpr=1

 

To answer your question of replacement carpet, I'd first suggest trying to clean it. It can be done with the proper equipment and some patients.