Based on recommendations I found in other threads, I bought a non-AC heater core from NPD, and it fit perfectly. I don't have any personal experience with the AC version, but they do list one on their website, and it is from the same manufacturer as the non-AC version:
https://www.npdlink.com/product/core.../194463/100664
I hope your leak is fixed because, you're right, it's loads of fun taking this stuff apart. I had a real hard time getting my heater box off the firewall because a previous owner glued it in place with silicone sealant.