Most research institutions, museums and local studies libraries will have map collections.
Many of these sell reprints of old OS maps produced by Alan Godfrey Maps. The central London area is covered on over 90 sheets, some at 2 or 3 dates. They currently cost 2.50 and can also be purchased online. Stanfords in Long Acre sells every kind of map including some historical maps.

The 2 major collections of London maps are at the British Library [site] and London Metropolitan Archives [site].
The latter includes maps previously held at Guildhall Library.

Maps available online include: First edition OS Maps [site]
1:2500 (including Central London) & 1:10560 (GB) OS Maps [site]
Historic Maps including Stanfords 1862 Library Map (with index) [site]
London & Environs maps & panoramas [site] Old London Maps [site]
The Crace Collection [site] Booth's Poverty Map [site] Snow's 1859 map [site]
Victorian London [site] Landmark (Lambeth's Digital Archive) [site]
Vauxhall and Kennington map list [site]
More map sites are listed on my links page

The Topographical Society have a website and have produced a book of 'bomb damage' maps. This information is now available online [website]
Goad fire insurance plans (1886-1970) are detailed large scale maps (40" to mile) but are only available for some areas.

Two books which include a large number of maps are 'The Times London History Atlas' and 'The History of London in Maps' see [booklist]

london-footprints.co.uk 2013