A Rare Bone is an item introduced in Generation IV that can be sold for a high price. It can be found while digging in the Underground in the Sinnoh region games.
In Pokémon Black and White, wild Dwebble and Crustle may rarely be holding this item. You may also find it in the White Forest while Britney is there. It can be bought for 30,000 from the market in Black City, however this isn't recommended as the selling price is 5,000 at Poké Marts or 10,000 from the man on Route 18.