If you cannot find a proper size on internet, maybe you could ask locally ? I know some folks from my country, when they had problems with uncommon size lenses, they went to watchmaker or company, that sells glass. It's not a big problem to cut a 59mm circle from glass sheet for any of them. If the glass thickness won't match, you can always change the o-ring to thicker. In automotive shops, they have a lot of different sizes/thickness o-rings.
Was doing some modding on my SR STL-V2 when the lens rolled off the table and crashed onto the floor. Does anyone have a spare lying around or perhaps direct me where to get it? It measures 59mm diameter.