I'm not sure anyone still makes them.
You can make your own with a stiff nylon or leather strap, two machine screws, two thin nuts, and four washers.
Disassemble the pump handle from the action bar assembly.
Drill two holes in the bottom of the pump handle, at the front and rear.
Attach the strap to the handle with two machine screws and the two nuts.
Use washers under the head of the screws and under the nuts inside the handle, securing the nuts with Loctite.