Brownells.com has what you need including 6-48 0r even better, 8-40 screws, taps and drills. That said, most gunsmiths and H&R won't drill and tap a 12ga shotgun barrel, I'd silver solder it on rather than take the chance of getting hurt or ruining the barrel, putting multiple holes in the thin chamber is risky, but I know it has been done. Warne makes a couple steel Handi-rifle rails that are made for the same barrel contour as the 12ga.
Tim