I think I had one show up once in the requistion options at the end of a mission. (Or was it a Chaplain?

)
I missed him because I did not have enough requsition points and at the time I didn't think I would not come across another one.
I am well beyond day 700, and it's only happened once (when I was given the choice, I picked a Purifier, while I got two Paladins through other events).
edit:
confirmed, I just got a Librarian from the requisition at the end of a mission (and got a Chaplain earlier)