You have 2 choices - rebuilt cores with steel sleeves. By now I doubt there are any original cores left.
There are also new calipers; I have heard a good rebuilt caliper is the way to go.
Lonestar is the supplier to most parts houses.Lonestar
There are 2 piston tryes: lip seal (original design) and o-ring.
The o-ring style is considered the better design.
The hardest part of 65 thru 82 Corvette brake work is bleeding.
I use a Motive Bleeder.