Why would you want them? New replacement breakers have not been available in years, used ones are outrageously expensive and not easy to source, also they could be 40-50 years old, some things need to die and Zinsco/ GTE Sylvania equipment is one of them.
You are not finding these because nobody sells this junk. If this is a misguided attempt to save a customer money by replacing the panel guts versus the entire enclosure, you *might* be able to find another manufacturer's internals that can fit your enclosure.
Another alternative is abandoning the original cabinet and putting a new one next to it, using the old cabinet as a splice box.