This rather limits tyre sizes that can be used, as I understand the tyre must be 10mm smaller than the mudguard.
You can run a 50mm fender on a 50mm tire, but you will need to raise it a bit, relative to the top of the tread, to ensure there is adequate clearance at the edges of the fender where they curve down. Also a 60mm fender will provide better protection from spray compared to a 50mm fender when used on a 50mm wide tire.
GB makes a 26" x 60mm wide stainless fender so if you really want a set I'm sure you'll find a source. I spent a fair bit of time tracking mine down.
The Planet Bike Cascadia ATB fender is a very nice plastic fender that is 60mm wide if you aren't 100% set on metal.
safe riding,
Vik
www.thelazyrando.com