Don't know of any websites which have them, other than TfL's which only has current ones. There are, however, quite a few examples on display at the LT Museum Acton if my memory serves me right (no idea if that's of any use to you mind!).
I haven't been in the poster store at Acton yet, but expect that there will be plenty of maps in there. It's a stop on the weekend tours, see here. I would contact the museum if you have an even vaguely convincing research request, they might be able to arrange copies or to take you in at some other time.