Yeah, the 4 Castform are on a cycle, so the original Castform will come back at the September equinox.
Ben is very much against the notion of exclusive things (and I agree with him), so aside from those random collectibles that show up during event arcs, you won't be seeing anything of the sort.
I just caught Sunny!Castform a little bit ago... forgot what thread, though. Think it was BraviaryScout's birthday thread.
I’m guessing the sunny one is more common then I thought because I just found another one and this time I caught it.
I have the Sun and Hail ones at the moment, I suppose I'll just have to wait for the others to be available