Good luck with that--it's been out of production since, as you said, the '80's.
Short of a lucky break of finding someone on eBay or similar site, with "new old" stock, you're not going to find a regular parts dealer with what you need. Beware of buying used keypads that old---good chance, if they're working at all, they're on borrowed time. Keypads have moving parts that eventually wear out.
You might try this site that specializes in electronic security, someone there might have something in a pile of old stuff.
https://www.doityourself.com/forum/electronic-home-security-systems-alarms-devices-87/
