I'm sure if you post a "Want to buy" in the "Want to buy HK parts and accessories" below, someone will be along to offer one.
These are offered on a regular basis in the "Want to sell" sub-forum:
WTS: G3/HK91/PTR91 Wood Furniture. SPF
Back when these were freely available, I had to buy several in order to get the two in nice shape I wanted. You might want to ask about what condition those offered are in.