Most of the time, watch band sizes are given by the width of the strap (i.e 20mm or 22mm). You (or any jeweler) can measure that dimension on the watch face, and then it should be super easy to find a compatible replacement. For the most part, the locking mechanism for attaching the band to the watch is very standardized (spring loaded barrel pin), and while they can be fiddly, generally they are fairly easy to work with.