Yes, there’s aftermarket for almost everything
… I can’t really help you with suggesting a WH muffler and I’ve never seen one installed on a WH PVC vent pipe. Over the last few years I’ve inspected many of the power vent water heaters and several noisy ones, but in most cases it was the motor, not the air.
However, to answer your question about up-sizing of the WH vent pipe to 3” diameter – Yes, it will reduce the noise because the exhaust gasses speed will drop significantly with the larger diameter of the PVC vent pipe. Install the reducer as close to the induced draft motor enclosure as possible (depending on your water heater size / BTU’s you may be able to go up to 4” in diameter but you have to check the manual).
Additionally, install 45° or 90° at the exterior termination of the water heater power vent pipe
to deflect exhaust gasses and the noise towards the ground (put rodents screen at the end).
I’ve heard suggestions that additional elbows along the run will further silence the noise but I’m not sure of that. If you decide to do it, use vent connector length tables that apply to your particular appliance.
Let me know if this helped or if you have any other questions.