Unfortunately, Original Tractor Cab Co. only makes an economical cab for the BX series, not the B-Series. Too bad, since OTC provides excellent customer support and that option would have been the best price/performance as it's easy to install and remove.
There's a few other commercially available cabs that will fit the Bxx20 series that you can look at (some of them are not very cheap though):
Manufacturer Unknown Soft-sided Cabs - $1, 245 and $1600
(Note: Some of these cabs require the ROPS Fiberglass Canopy option to already be installed. In addition, several dealers sell this brand of side-sided tractor cab for around the same price, just Google for more.)
Curtis Industries WorkPro Bxx20 soft-sided cab - $2,750
(Note: The rear hard panel kit doesn't work with the backhoe option.)
Sims Cab Depot Yukon Hard-sided Cab - $3,255
(Note: Includes one hard-sided door, one fixed side panel, hard-sided removal rear window, and flat windshield.)
Sims Cab Depot Indy Hard-sided Cab - $4,062
(Note: Includes two hard-sided doors, hard-sided removal rear window, and curved windshield)
Tektite Kubota B2320 Series Cab - $4,700
(Note: One piece deluxe cab.)
Laurin Eclipse Cab - $5,566 (EL), $6,116 (ES), and $7,784 (EX)
(Note: One piece deluxe cab. Comes in three different configurations.)
There's a number of European cabs available as well (
such as this one for $4,568), but I don't know if they're imported into the US by any dealers.
Out of that list, even though I went with the
Sims Indy cab for my B2620, if I was trying get something good enough to stay dry - I'd definitely go with the Curtis Cab for the money (plus it's made right down the road from me in Worcester, MA.)
And if I had to do it all over again, I'd probably just buy a B3000 with Factory Cab and forget about aftermarket tractor cab options all together!
Good luck!