Slaters, Wills or Evergreen probably have something.
I like Evergreen, it might be expensive compared to the others, but it is
If you have problems finding it, "The Model Shop" on City Road, London have
it. (no connection, except occaisional customer)
The latest Hornby club magazine has a DIY version. Cut the flat bottom out
of a takeaway meal container (the thick foil variety). Place the flat piece
of foil on a piece of card and draw parallel lines across with a ball point
pen. You cna then cut out recangles and paint them rusty colours.