H&R is the only company making cartridge based barrels(with lugs) for these guns. There may be a few doing custom stubbing. Some smiths will rechamber them for us, and a handful are capable of reboring.
If you want an AI, or in the case of these Handi's an "Improved" chambering - either rent a reamer or send the barrel out to a smith. Since you cant turn the barrel back the proper amount for a crush fit, a true Ackley Improved with a rimless cartridge is nearly impossible unless you can clean up the prior chamber completely.