Hello Alex,
All softtop frames fit. There are differences in details but generally you can swap in any 113 softtop fame.
Since the California coupé was not designed for the softtop, there is some specialized work necessary to put the softtop frame in, e.g. constructing the rear latch and more but basically you can get it working.
Softtop frames are not easily to come by. You can ask Gernod Nisius from SL Tech in Aurundel, Maine; he certainly has several frames. However, those are pricey but overhauled.
You should also ask the other sources with vendor-like characteristics: Dan Caron (Dr. Benz), Tom Colitt, K&K, and others.
Just keep your eyes open.
Good luck
Achim
(who finished hunting for his softtop frame)