You'll see bean sauce called for in many Asian recipes. It is often sold in the Asian foods section of the grocery store. But if you can't find it, use hoisin sauce in its place. Hoisin is sweeter, so decrease the amount of sugar or honey in the recipe if there is any.
You may also see black bean sauce and chili bean sauce in the store, and you can use either of those in place of Chinese brown bean sauce, although both of these will be a little saltier and the chili bean sauce will be significantly spicier.