There are several fiber optic sights available, for a quick and easy solution. Make sure to check the width of your rib on the barrel. HiViz and TruGlo are 2 companies that come to mind.
If you're set on getting another bead, checkout Brownells for an 870 replacement. You have 2 options there...drill & tap another hole for the new bead, or back out the snapped off screw-base from the bead that broke. These are not impossible as DIY projects, but a gunsmith could do it fast(er).
Good luck
Last edited by JoeA; 02-07-2019 at 10:33 AM.
Reason: typo