You make your own. It is very easy.
You use 3/8' plywood
Make the cut out you want with a rip saw. Now cut the board to size, with a skill saw, or table saw..
Mount the hardware.
Paint the outside of the frame with mat black blackboard paint.
Now cut the fabric to size, and staple is with a staple gun.
You can get the hardware and speaker fabric from Madisound, and I suspect a lot of other places.
I doubt there are any custom builders.