David Booth's The Tate Gallery by Tube (1986) proved to be one of the most popular posters and has been sold around the world. It was an advert for the Tate Gallery and shows a tube map being made from lines of paint squeezed from a paint tube with the tube logo and the word Pimlico on it (Pimlico is the nearest tube station to the Tate Gallery). By the 1990's advertising was being sold more agressively by TDI (now Viacom outdoor), so there were fewer of these "free" spaces. Even so, market research showed that the public liked these images which weren't hard sell and provided something everyone could enjoy. Now the London Transport museum is home to the great historic archives of the tube poster and is also the principal retail outlet for these posters.
I also framed the complete underground map next to the Tate Gallery poster.... it was actually really helpful when I had guests staying - they would decide where they were going and then sort out their route before leaving the house!
Once I started delving around, it seems that there's a number of fabulous posters and signs for sale. Take a look at these tube station signs.... they're not that expensive either:
I really would like to own one of these old bus signs - remember the old London red buses - they were called Route Masters and you could jump on the back? Well these are cloth signs that were placed at the very front of the bus, placed on rollers so the bus driver could change the destination with the twist of a hand:
Of course, the New York subway posters always look great: