I did a Q&D search of the transmission section and didn't see any specs for the drain plug. I did see the plug in one of the drawings but I don't think it gave a p/n, just torque specs.
I tried
Courtesy Parts and couldn't find the drain plug at all. I'm assuming the problem is on my end as I can't imagine CP not selling them. See if you can do better.
Somebody on this site is bound to know. Maybe they will speak up soon.