Several vendors, but they all probably originate from only a few factories in China.
Having said that, the technology itself isn't as advanced as you might think and I'm sure there are companies in other parts of the world that can build CF tubes. The biggest cost is having the mandrel made, but if you want to stand out from the crowd, you can certainly have your own taper made with the wall thickness you prefer.
In my experience the key factor is the QC in the factory, so the tubes you get sent are as straight as humanly possible, so finding someone who will do that for you and be cool if you reject a blank is important.