I'm no expert on pistons but it looks like those are the original 'skeleton' style pistons with 3/16" thick compression rings and 5/32" thick oil rings. You could try Gary Wallace (
http://www.earlychevyparts.com/) for rings that will fit however, my guess is it will be cheaper and easier in the long run to replace the pistons and rings as a set with 'modern' replacements from Egge or similar.
I'm sure others with more experience on this will chime in. I for one had my engine bored and sleeved back to standard and used Egge pistons and rings with no issues.
-Tyler