The XS brake parts are getting difficult to get now so here are some tips on how to get parts. The caliper pistons are not listed at Yamaha at all so if you need them, you either get a second hand caliper in good condition, or you find another source for them.

I tried various suppliers but found them to be quite expensive. The best way to get them I found was to either make them yourself (If you're hand with a lathe), or have an engineering workshop make them for you.