I was also chasing a single ring for a 250 piston, but 71.5mm. No luck. I think they are 1.25mm thick for the single ring elko piston.
A 71.25mm ring was available from Al Beuhner in the states. Give him a try.
Else your only option is a new 71mm piston, or just go the next oversize at 71.25mm. Both available on Al's website.
Regards,
Adrian