Try the vendors. Most have rebuilt calipers and will take yours as a core. The caliper pistons are nickel plated low carbon steel; water in the brakes from age & not changing fluids penetrates the plating like a neglected bumper and you get rust pits. The pits cause leaks even with a new seal; honing the bore doesn't help.
If that's the problem, grinding and replating the OEMs, or new stainless steel pistons will fix the problem. 10 yrs ago I had to make my own but I understand there are now some replacements available- don't know which method they used.
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