Its not rare. Its basicaly the same as the 2T or 3T dizzy, but a bit longer so the main head of it clears the carbies. I think the shorter 2T dizzy may still fit, not sure.
2TG dizzies ARE hard to find. The points style Dizzy's are the most common, The elctronic ones are rather scarce. You can be lucky and get them from the importers for around $150.
The 2T and 3T dizzies DO NOT fit due to their fine adjustment knob fouling on the intake manifold and preventing correct timing adjustment.