I buy from a Locksmith supplier. You have to be bonded, and there is a credential process you have to go thru to buy from them.
They're the most consistent and reliable. However, this stuff is getting old and even they have trouble getting stuff.
I've also had luck finding old lock build kits at swap meets. Hershey swap is the best for that kind of stuff.
Ebay has also been a decent, albeit spotty, source as well.
Also, I've been known to stop at old locksmith shops, and ask what obsolescence they might be willing to come off of.
I've bought most of my NOS hard to find keys and a stash of 62-65 Mopar door locks that way.