If I were you, I'd start with a prepared bone blank from any of a number of online sources, such as eBay, C.B.Gitty, and the like.  The dust that is generated is smelly and dangerous to breathe, so use a mask or respirator when you sand or cut bone. Same goes for mother-of-pearl (MOP).

I purchase bone for guitar nuts and saddles from Stewart-MacDonald. Dan's eBay source looks good too. A lot depends on the size of bone you need for the dulcimer nut and bridge and how much you are will to work it down. Some folks don't like the odor created when cutting, filing, and sawing bone. If you really want to start from scratch you can probably get a beef bone from the butcher at the grocery store. You'll need to boil the meat off and then let it dry. To get a nice white color you will need to bleach it as well. Then you will have to cut it to size and shape it. Have fun however you choose to tackle this project.

Ken

"The dulcimer sings a sweet song."

 

I get my blanks off ebay,  bone blanks