You'll have to buy another Handi or a used SB2 frame in the classifieds or elsewhere. You can't buy a new frame(receiver) from anyone, they aren't sold that way. I wouldn't plan on sending it to H&R for fitting, they may be able to IF the underlug is long enough to be cut to fit, but if it's already too short, they can't. But you can fit it if it doesn't fit already(which it might) following the barrel fitting info in the FAQs and Help sticky.
Tim