I too have an L1200, and it came with only one neg holder although the femoneg carrier was there. They come up once and awhile, but---lately Stone mills has had many on ebay...and others in stock, so I WOULD contact them.
There's another seller now with a lot of durst stuff as well.
It's taken me a year, but I now have all the masks I need.
I could also use the PDF manual if Mr. Callow would be so kind...
Matt's Photo Site
"I invent nothing, I rediscover". Auguste Rodin