A few years ago someone was scrapping a BX on ebay and I asked if he had the locking ring.
He sold me the locking ring for £5, but sent it still attached to the upper filler neck. That filler neck was the wrong one for my car, so I took the locking ring off that filler neck and put it onto the filler neck on my car.
So, whilst you cannot buy a locking ring, you can always ask for one, and if someone sends it attached to a filler neck, reomove it.
NOTE: it was a bit fiddly to remove, and also fiddly to put on the filler neck on my car. I hope this helps to make it clear what I've kept on saying.
