You can make your own, if you can weld exhaust pipe, but anyone selling one could be liable for a rather hefty fine. Right now the EPA has cracked down hard on the Diesel Brothers, and some other guys that have You Tube channels for modifying their emissions systems. I think the Diesel Brothers were fined $850,000.
I had a friend that owned a Midas franchise, and used to make them up, but the threat of the EPA busting anyone making test pipes scared him away from continuing to make them. This was in the 80s, not recently. This was the time the test tubes went away, and disappeared.
The other avenue is to change the early 2 hole flange on pre 74 headpipes to a 3 hole flange so they can be bolted to the later manifolds, then retrofit an early dual system to the later chassis.
The old convertor can be gutted, and empty the pellets out too. You have to be able to weld the plug back in though. I've probably known of these ways around the catalyst since the 70s, I would never do them though.....
