The muffler size is determined by the chassis maker. if it's a Freightliner or Sapatan, you can call and ask what the size needs to be (have your VIN handy). You can probably buy from them as well - sometimes they have excellent prices.
Spartan's online parts sales are through this website:
https://www.rvchassisparts.com:8443/aparvchassis/runApp?id=5
Freightliner Custom Chassis can be reached at 1-800-FTL-HELP
Once you know the in/out diameters and length (and perhaps the outlet angle needed), you can shop by size for the best price. Even ebay and Amazon are good bets. There are several major brands of diesel mufflers, e.g. Donaldson and MBRP. some folks go for the widely advertised Aeo brand, which touts better performance (I'm skeptical).