Greg-
Contact Jeff Kane at the Train Tender http://www.ttender.com/. Part number 2623-17 ($0.75 ea.). Part number for the door hinge pin is 2623-18 if you need that as well ($0.25 ea.). I've never installed one so others can weigh in on ease/difficulty of installation. You may have to detach the vestibule from the shell.
Dave Branch