They are supplied by Moss Motors via the manufacturer in Argentina. The boots ("gaiters" in Britspeak) are available from Moss of many other sources including NAPA. Treating them with a good rubber protectant will extend the life. BTW, if you have a MK I while the boot is off you can remove the steering lock limiters, they are split alloy rings held on with a zip tie. On MK I cars the tires may rub at full lock without them, but your turning radius will reduced.
They have shorter pinions shafts for V8 conversions. Clive Wheatley, B-Hive, Brown & Gammons, MGOC
As in MGB V8 conversion
… or just knock out the pinion shaft, cut to desired length and get it splined for the UJ