Pet shops sell cleaned and sterilised pieces of bones for dogs. £1 a big bit last time I bought some.
Failing that, and this is a good time of year for it; visit the butcher, and then boil the meat off the bone for soup and bury the bones in a pile of leaf litter. The worms will move in and clean the bones off for you in a couple of weeks
I do it by putting the bones into my compost heaps, and the brandling worms do a sterling job
cheers,
Toddy