FujiGuys have officially confirmed that the X-Pro1 is using 19mm diameter diopter lenses
Here are some products:
Cosina/Voigtlander diopter correction lenses for
Bessa R2, R3, R4 and Bessaflex single lens reflex.
Nikon FA, FM, FM-2, FE and FE-2, X-Pro1
Zeiss Ikon 35 and some Contax cameras.
They come in various strengths to suit your eyesight.
Nikon +0.5 Diopter for FM-3A, FM-2, FM, FE-2, FE & FA
Germany: Zeiss Ikon Sucher-Korrekturlinse