The high upfront learning curve for 3D printing is somewhat of a barrier to DIY. However Shapeways/Thingiverse/etc will print from a large library of models an

The high upfront learning curve for 3D printing is somewhat of a barrier to DIY. However Shapeways/Thingiverse/etc will print from a large library of models and ship it. I suspect the big issue is the custom fitting. Assuming a standardised approach to custom fitting exists then standard models could be made available for purchase without insane amounts of effort. The fact someone is making prosthetics out of lego strongly suggests that such a standardised approach exists.