They are out there, but as you say, rare. I still miss this Stella mixte I had. Quite a looker. It was mostly for decoration till I gave it to a bandmate during my slacker years. I think the frame was Vitus, but can't recall absolutely. Components were nice -- Campy, Stronglight, Jubilee, etc.

That bike was a rocket. Super quick. Riding a quality lightweight mixte opened my eyes a bit, and made me think perhaps all frames should be mixtes. It's in some ways a better design than a standard diamond frame.