Couple of issues I can help with. First off, are you in need of the cast iron adapter that mounts the transmission to the transfer case? This is commonly known as the adapter. Or are you in need of the 10 splone couple that physically connects the transmission output shaft to the transfer case input shaft?
If you are in need of the 10 spline couple for the T221, it can be found, but will be very hard to find a good one. Search this site for " Rockwell T221 lives" Vero67 lists the new seal to use with a NP205 10 spline shaft. The seal will make up the difference in diameters between the shafts. The couple will need shortened and the teeth will need to be ground back/modified a bit. I have been having a lot of T221 fun as well
Bruce