I would second what Xort said. Carbon fiber has a nasty tendency to fail without warning. It also has a problem if you scratch the laminate, since that can cause a potential failure point. When Carbon Fiber bicycles first came out, there were some really nasty failures due the the frames being scratched in collisions and then failing without warning a while later.
I heard CF was the best for rudder posts because it does not suffer from corrosion and it is stronger then steel. For added strength, foam can be used to fill the inside. But I am not an expert. I will look into it.
I found a source called Carbon Fiber Tubeshop and
Forte Carbon. Does any one know anything about them?
By the way,
One other point that you may not have considered... Graphite, the primary component of carbon fiber laminates, is very high on the anodic scale, and using a carbon fiber rudder post may put nearby underwater hardware at greater risk of galvanic corrosion. IMHO, really not worth the expense... Also consider, if you sail anywhere remote, a CF rudder post is going to be far more difficult to repair than a stainless steel, bronze or aluminum one would be.
Where are you located? If there's a really good boat yard in your area you might ask there. By "really good" I mean, do they build custom boats? If they use a lot of high tech materials so much the better? One such place is New England BoatWorks in Portsmouth, RI (New England Boatworks Builders of advanced composite sailing and power yachts) -- they might be able to steer you in the right direction. Call 401-683-4000 and ask for Ian.