As Ben said, the only source for a 357Max barrel would be the used market, preferably in the H&R Classifieds here, H&R only offers 357mag accessory barrels, they have in the past made Max barrels, but not for a long time. You can make your own Max real easy if you can get hold of a barrel either from H&R or used, see the sticky Ben pointed out, there are three waiting on the list at this time, the reamer is inbound to me right now. Be aware that 357mag or Max barrels go fast if and when they become available in the classifieds here, so be patient, some members have waiting months for one, you'll find barrel fitting info in the FAQs if you wish to go with a used barrel.
Tim