1963 Dart L6 Heater Core

DO it yourself. It has only FOUR nuts holding it in with a couple of cables to disconnect. I had my heater box out in less then 15 minutes, cleaned up re-gasketed core replaced and back in in well under an hour and I'm a retired and disabled wore out turd. For 200 bucks, they're bending you over the desk without a kiss.
I already have the whole heater core out but I'm not finding a core replacement available online or at local part stores.

Could you direct me to a core I could purchase online to change it out? I'm having no luck in searching the # online, everywhere seems to be sold out or not available.