https://www.dynaflexproducts.com/ Largest MFG. of class 8 exhaust components in the U S A. Measure your, go to any class 8 truck shop, they can order one. A resonator would be a good choice, less back pressure, not much change dB rating. Lots of trucks have one inside their exhaust stacks.
The decibel (dB) is a unit used to measure sound intensity and other physical quantities. A decibel is one tenth of a bel (B), a unit named after Graham Bell, the inventor of the telephone. Its logarithmic scale is convenient to represent the entire range of human hearing.