Hi Guy,
I was on this quest until recently.
I actually just got my purple teacher from Cundco as a spare parts request, but it may have been one of the last ones available.
I spent a very long time searching the internet for a black teacher, contacting lots of eBay and BGG sellers, etc. The thing that eventually worked for me was finding a picture of a black teacher on an Austrian games review website and contacting the person who had written the blog!
If you wanted to try a similar tactic, I'd suggest starting at BGG and contacting any users that have pictures of black or purple teachers. For example:
(CARC_Zoner was once active on this forum as well, but I couldn't get in touch with him when I tried)
I hope that helps as a basic strategy.
Finding the teachers was definitely the most difficult bit of Carcassonne collecting I have done so far. Even with the really rare expansions you can always fall back to paying way too much for them on eBay if all else fails, but the teachers are very elusive! Good luck!
JT
EDIT: I was just looking through my messages from when I was trying to find a black teacher and I remembered that I did manage to find another possible source of a purple one along the way! Try contacting the BGG user
Joshx, I believe he still has the purple teacher pictured here:
https://boardgamegeek.com/image/1159639/carcassonne-school