Unless you are doing a concours level restoration you don't necessarily need a caliper that actually came from a CX500E. Honda used the same calipers on a number of models and I believe they used different part numbers for identical calipers that have the HONDA name in a slightly different location and/or had the paint sanded off of those letters so that they appeared in silver.
Note that they came in several piston sizes as well as variants like the GL1100 Aspencade's wider calipers (to accommodate their thicker automotive style ventilated rotors).
That means if you don't care about inconsequential differences in location or finish of the logo you can use any Honda caliper with the same piston size (except those made for the Aspencade).
The piston diameter is indicated by the number stamped or cast into the end of the caliper as shown in the pic below (if your original left caliper is missing the size should be the same as the right one).