You'll have to measure them, genuine William's caps don't fit the prima ones, I had a set of each many moons ago..
If I couldn't find a cap I liked, I'd get a plastic ball a close enough size, cut it, paint it and glue it on..
The Williams original caps are (old) a rubbish fit, I've actually put a blob of glue on to stop each £40 or so cap falling out.