Do not assume that because you are interpreting the 18th century (or 19th century) that all your accoutrements must date to that period. Items handed down through the family, or items aquired at an early date will still be in use at a later date. Here I will show you some belt and strap buckles dating from the 16th century through to the 18th century.
If you can't find the appropriate design in a modern made buckle, then avoide modern metals & coatings. Go for old saddle and harness buckles of brass and iron.