Last one I saw is made by Mossberg, but it's made as part of a Model 500 series forearm.
You can easily make your own from a leather or thick nylon strap, two machine screws, two thin nuts, four thin washers, and some Loctite or epoxy.
Disassemble the forearm from the action bars.
Drill two holes in the forearm.
Attach the strap to the forearm by putting a screw and washer on the outside of the strap, then pass the screw through the inside and put a washer and nut inside.
Use Loctite or epoxy to secure the nut.
If there's not enough room inside for the nut, reverse the screw with the head inside, and the nut outside. You can countersink the holes inside to bury the screw head and give plenty of clearance.
You can bolt both ends of the strap on the forearm or form a loop and bolt only the front end and leave the strap loop loose.