This is not just a standard pin nor a roll pin. These come in some rebuild kits.
You have two choices: Call on Monday Borg Warner and see if they ship you one, depends on the reperesntetive they do that (don't ask how I know -hehe)
and/or
register here
Vintage Mustang Forums and ask the boys. There are tons of dead T5's in that forum and might have a spare...
Good luck with your hunt!