DH has bought the H buckle on it's own from the boutique when the old one got scratched, I don't know if they sell the other buckles on their own though? If you check out Hermes.com they have a few different styles.
I was also told one has to buy a belt and buckle together. After that one can purchase just the strap. I fail to see the logic other than to make someone pony up for $$ if they need or want a new buckle.