When I ran a railway station, on a main line into London, we could never get hold of as many bus route maps as we wanted. That's not because our passengers all wanted to use London buses, but simply because people realised that they were excellent, free street maps of the city.
The bus route maps are also useful for anyone who wants to see the true routes of the Underground lines, because those lines are overlaid on the street maps.
The Transport for London enquiry offices, at main railway stations such as Liverpool Street, should be able to provide you with the full set of the printed maps (SE, SW, NE, NW and Central), which each take up roughly the equivalent of four A4 sheets. You can also download the maps but, inevitably, compressing them onto a single A4 sheet will make them harder to view:
http://www.tfl.gov.uk/gettingaround/9440.aspx
Chris