Laurin does make a nice cab for the B3030. I do not for a second believe they make the factory cab for Kubota. I think Jodale Perry makes a nice cab also.
Sims Cab Depot cabs are much "cheaper", but not a whole lot less expensive. Not much bang for the buck, in my opinion.
I did find a company that makes cabs similar to the Curtis cabs that are more for just winter installation and summer removal, however, they make production runs in mid to late summer, so they have nothing available now.
Curtis does not make a cab for the B3030.
This same issue has made me realize that if I want a cab badly enough, I will most likely be trading in on a new cabbed machine.
I am waiting for the new B3000 model since you can install the new Kubota BH77 backhoe on the factory cab model.
If you don't currently have/need the factory backhoe or you have the Woods backhoe, you may be able to find a leftover cabbed B3030 for a decent price, and might make a trade worthwhile.