Wrote up a quick tutorial/technique guide to making a new bone (say, I dunno, a canid's jawbone :P ) look old or fossilized. I used this technique for my own prop jawbone for my Lady Solas cosplay after much searching on leatherworking forums (it's apparently a technique used by knife makers who use bone for the handle a lot). It was pretty easy and actually cheaper/faster than sculpting it and painting it. I already had most of the supplies (leather stain, applicators, varnish, etc) since we do a lot of leather working projects.
I used a juvenile coyote jawbone and i got it from this shop on etsy (all of their animal parts are ethically harvested from roadkill etc and not killed for taxidermy/trophy purposes).
Not shown- we also drilled a hole in it (obviously using correct protection b/c bone dust is nasty stuff) and then I used artificial sinew to create Solas' necklace. Once I was happy with the sinew, I stained it brown/red with the leftover stain.
Tandy Leather carries all of these supplies - I prefer Professional Waterstain because all the colors have a slight metallic sheen to them, but you can also use Eco Flo if you want a more matte effect (I used Eco Flo bison brown, and Professional Water stain Gold & Red in this project). I also really like using their little applicators- A big bag of them will last you for awhile. If you don't want to use the applicators, you can always use an old rag and apply the stain that way.