Your best bet is to go to your local arts & crafts store as others have mentioned. I have seen several different weathering and patina faux finish kits.
Or, you might also try weathering it yourself but covering it with a "black wash" (2 parts thinner, 1 part flat black enamel paint), do the same with some "green wash" but not as heavy as the black and then cover the buckle with a dull coat spray. Rub some of the dull coat off on the high edges after about 30 minutes dry time.