They do not exist.
I think there's not a need for a carbon quill stem. Since most carbon components are used on high end bikes, high end bikes use threadless fork and headset...so a quill stem is not necessary. Or it could be stress load issue.
Carbon Fiber bike components were still cutting edge and somewhat experimental when quill stems were being used. By the time carbon fiber technology became strong enough to consider doing such a thing, theadless stems (superior technology) had taken over. Still, it took a few years for threadless carbon fiber stems to come about.
People don't like flex in the stems and bars, I think carbon quills gonna feel like riding a spring.
Also, production cost (plus development of manufacturing processes) could be huge ... for demand fairly low. I mean it's gonna be uber expensive and most people would opt for age-proven aluminum (or CrMo) stems for reliability (at the cost of minute weight saving). This leaves very little incentive for companies and factories. ... You ain't polish my shoes for free, right? Nothing is free.