Hatsan USA will sell you a 135 stock if they have one available.
135 stock fit one of my bud's 125.
The thumbhole stock on the 125TH that I recently purchased was cracked when I got it, and it didn't take long to break completely in two! I managed to repair it using J-B Weld Quick and and a series of reinforcing pins that I made from pop rivets. It's held up for about 50 shots so far, but in anticipation of another failure I asked Hatsan USA about the availability of replacement options. Here's what I received from them on February 25, 2016:
Inletting I can do. Knowing how to order a Boyds stock, which one would fit, I would need lessons.