I'm about to start a full brake upgrade to my 97 DX, I have all the main components but I am trying to rebuild the calipers and can't find the rear piston's. I could probably save them but they have some heavy rust and worse, pitting where the boot connects. None of the local auto parts stores carry it and online searches have not been very good.

I will be going to the dealer on Monday to see if they carry it but if not then is complete replacement my only option?

 

The dealer is likely going to be your only bet for pistons, even then they are almost guaranteed to have to be ordered in.
"most" people would just end up replacing the caliper.

You can order them here for $47 each. #9

Honda Automotive Parts

 

Dealer wanted $70 each... looks like I'm getting new calipers

 

Honestly, you might as well.

if you were just slapping seals in, rebuild, but replacing such an expensive part you might as well go reman/new.