CC has it pretty much right.
The mast manufacturers do not sell replacement halves.
Powerex was replacing halves under warranty for a while, but they have stopped
Look on the used market and see what you can find,.
If you replace the mast I'd look for something besides a Fiberspar QT for your 9.5 m2
Retro. If I recall correctly, the Fiberspar QT bend characteristics were somewhat less than ideal for the Retro Sails.
I somewhat disagree with CC on the "masts get old and eventually break".
I have some really old carbon masts and I take good care of them.
I don't think they will be breaking anytime soon.
Abuse and direct sunlight generated hot spots are the main "mast breakers".
Used as they were designed, and only bent in sails, they will last a very long time.
Hope this helps