There's a local guy here who has a lot of older (new) parts. Well, what didn't get blowed away....shoot me an email (dhadley50@aol.com) and I'll get you his phone number.
In the mid-seventies I tried to get replacement rings for my '61 35HP Big twin. I was told they were discontinued, adn the replacement was the newer "Pressure-back" pistons and ring sets. You may have the same issue.