I needed the same item for my transmission and was never able to find one. I contacted Tremec and they said that part is no longer available. I found a different Tremec boot from a seller on eBay, fabricated a new mounting plate for it and it seems to work well. This is the link to the eBay ad:
Tremec Replacement Rubber Shifter Boot Cover T5 T56 Magnum TKO 500 600 TR6060 XL | eBay
For the new mounting plate I just cut a round disc from 3/16 thick aluminum plate. I made the outer diameter just large enough that I had to stretch the new boot onto the plate and that the boot would clear the shifter tower when bolted on.
Ted