Many moons ago I inherited a 30.30 H&R single shot. In the ensuing years I have heard they came with, or it was available to get a .410 barrel for this little beauty. Any ideas??
egunparts.com is a great place to start. The site is actually numrichs gun parts, but they sell original parts for old guns. Their inventory is constantly changing so you may need to check back if they don't have it the first time. They will also buy old guns for parts.
Contact H&R directly. My dad had a NEF single shot and sent it back to the factory where they matched a 410 and 20 gauge barrel for his 12ga. NEF is owned by H&R now so that is the route I would go.