I am in the same boat and have been looking for a 3pt kit for my
B21. From a search of the forums it looks like a kit from a
B26 will fit. Here is a 2010 quote from BruceWard:
If you are interested in getting the official Kubota parts, there is a 3 point hitch kit for the B26 that will fit the B21 and costs about half the price of ordering the B21 parts individually. The 3 point kits part number is B3941.
I tried searching for the kit on Messick's and TractorSmart but did not get a hit and my dealer had no record of it. Please post back if you are successful. Good luck. Here are the links to past discussions:
http://www.tractorbynet.com/forums/kubota-owning-operating/185907-kubota-b21-3-lift.html
http://www.tractorbynet.com/forums/kubota-owning-operating/166628-b21-top-link-lift-arm.html#post1912129
P.S.- I am planning on getting a 4' Gannon.