Aye Up Forestfella,
This company - DesignGo -used to make stainless steel roll pin buckles - don't know if they are still on the go - let us know if you find them.
Or - I believe that the Austrian surplus waist belt is canvas with a brass roll pin buckle, if you don't mind the eagle insignia might be worth a look.
Just in case you are not aware of it - beware of nylon/polycotton belts with a roll pin buckle - I have found that after a while the material becomes polished through friction and the pin doesn't grip properly.