I am not aware of anyone who makes after-market barrels, but there is a gentleman by the name of David White (his user ID on ShootersForum is ASSASSIN) who is a very talented gunsmith. He has done some amazing things with H&R actions/barrels using a barrel-stubbing technique that results in remarkably accurate guns. He has done some work for me and a couple of my online friends and comes highly recommended for the quality of his workmanship, quick turn-around times and most importantly, his friendly, courteous nature. Like this website, David is grounded in Christ and it shows in all that he does. Whether or not that really matters to you, he is the only gunsmith I've ever worked with who I wholeheartedly recommend to others. The 357 Maximum is one cartridge he has done a lot of work with, in fact he just built one for his wife to use for hunting this fall.
I believe H&R will only sell secondary barrels that they themselves have fitted to your action, so you are unlikely to find any used barrels. They probably would not fit your gun properly, or be safe to fire, if you did.
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