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I don't know about bone, but wood you can get at woodworking stores or maybe certain hardware stores (probably not exotic stuff). Metals you can get at hobby shops. Plastics you might also find at...

You could use plastic, wood, bone, brass- Some guys even use a key to shape a spacer from. Anyone have that link showing using a key for a spacer? I never really glue my spacers in, just make sure...

Leon, I've also seen a lot of 3-pins on smaller than 6/8 DD's. Anyway, I like the three pin look as long as you do it very carefully so that the side placements turn out even.