I've used bone, horn, hardwood and rawhide for reinforcements for self nocks, all worked well. You don't have to buy anything special. I used rawhide dog chew cut into strips for reinforcement spines. Got that idea of using rawhide from Glenn St Charles', "Billets to Bows". Heck, you could use popsicle sticks for the reinforcement.
These cedar arrows have rawhide spines...
...and these tapered ash arrows have osage spines...
...these hill cane arrows(by Art Butner) have maple self nocks added to the cane...
...these hill cane arrows have self nocks with no reinforcement and no plug in the center hole...
...and finally, this aspen arrow by Art Butner has walnut, 4 point footed self nock and foreshaft...