I'd recommend checking out restaurant and butcher equipment outlets. Expensive but they often have second hand grinders that are quite affordable.
I have to tell you though, we have a table top industrial grinder at work and it starts to overheat after only 7-8 minutes of grinding bones. I can go an extra 5 minutes or so before letting it cool down if I alternate chicken necks with meat. We have another giant floor model one that does bones easily (but you'll have to remortgage your home for one
) and even with that one, I have to sharpen the blades after grinding.