I've had some made over the years.I just love the HW 55 Tyrolean stocks so I had one hand carved for my Beeman R7.
The original poster asks for aftermarket stocks. The responders assumed wood stocks. Now, I know that for asking the following question, I might get strangled, hanged, beheaded, and ultimately finished off by an air rifle firing squad,... but here goes....:Where can I buy synthetic stocks to get rid of a wood stock? (Like for a Diana 56TH, for example.) OK, now you can execute me. Please, use big bores only.
Though I think your sanity should be questioned doubt the vintage area is the best place to ask?