A proper vintage buckle with the sun engraving will set you back +400 euro but the search can be long. There are also many many copies floating around so caveat emptor.
TAG Heuer issues new Heuer buckles with a B engraved for some of their Heuer LEs. These are also 16mm and a very nice fit to any vintage Heuer. They can be ordered through ADs and should cost around 20 euro. Part number is FC1042.