This joint is referred to as a companion flange. It was replaced by a universal joint in production years, later on.
Do you need the oil seal at the tailshaft housing, that the companion flange slides into? Or are you looking for a grease boot to fit outside of the companion flange, that goes over it to the driveshaft?
As far as I know, nobody makes that boot, that I am aware of. What people are doing, or that I have seen done, is used a CV boot repair kit, that has a metal flange inside the boot, with bolt holes that allow you to put the boot around something in place and bolt the two halves of the rubber together, around it.
This would be a parts store run with a tape measure, type of gig. Try Car Quest or Napa.
If you need the tailshaft housing to companion flange/ universal joint yoke oil seal, the parts store may also have one. If not, this is the part;
http://www.brewersperformance.com/proddetail.asp?prod=PS515