Welcome aboard. Midwest Gun Works has the parts that make up the rear sight assembly. Don't know where you might find the complete unit already assembled. Have you checked with Browning?
Browning A-Bolt Shotgun
The parts listed here are specific to the Browning A-Bolt 12 gauge shotgun model unless otherwise noted. Click the link above to identify your firearm's production date and if it is a Hunter or Stalker version. Find many OEM parts and aftermarket accessories like recoil pads, stocks, firing...