This page has nearly everything you need to know:
http://hr1871.com/Support/accessoryProgram.asp
For more information (including "insider" information), you can head over to the
Greybeard H&R forum, if you'd like.
For good measure, I'll mention one thing...
Used barrels are a great way to get what you're interested in. However, you'll usually run into one of two situations: 1. It doesn't lock up tight enough. Or, 2. It's way too tight, and doesn't lock up at all.
For case #1, the barrel
might be suitable for use if it's shimmed (but there's no guarantee).
For case #2, the barrel can be fitted to your receiver. Whether you want to do it yourself, or through a gunsmith, is up to you.
But, the easiest method, that will include
warranty support, is to go through H&R and get a brand new barrel fitted at the repair center.