Again, not exactly a convenient source bearing in mind your location, but Sprint Manufacturing in the UK also stock OEM Nissin components. Have a look (at the replacement caliper 27mm piston section) at their comments regarding the use of pattern versus OEM.
If you do end up having to replace pistons rather than just the seals, it may be worth springing the extra for geniune Nissin parts.
Sorry I can't help you with any cross reference with other Manufacturers. As I'm sure you're aware, these Nissin items must be used on loads of different bikes.