I'm not sure about a carbon tube source. I'd have to look around just like you will. I'd try Troybuilt or TNT Landing Gear first.
You're not going to save much weight with a carbon stab tube, but if it costs about the same as a replacement aluminum tube........ why not.

I still would not pay $200 for new stabs. I'd glue in some additional ply to make those stabs work. All it would take is a piece of ply the right thickness and width, that has 1/16th ply on both sides of it that overlaps and glues to the original mount.
It will be a little tedious to get it in place, but not impossible. I'd definitely do that before shelling out $200.

Jim