The muffler on my Suburban rusted out, and you can't buy a muffler from GM...it comes as part of the "assembly" that includes the right CAT, front pipe and some other stuff for the bargain price of just under $2000.
I did a ton of research on this because I didn't want anything that droned while towing, so I finally went with
Flo-Pro Exhaust. Their tech support was very helpful, pointed me in the right direction to make sure I got the muffler that matched the stock sound. Got it online for $150 and paid the local muffler shop $150 to weld it in. They have some models for your truck, you should give them a look before you pull the trigger.