Are you talking about 'old' (ie antiquarian) maps, or just tatty nlrs ('no longer requireds') ?

For the former, ABEBOOKS can turn up some surprises. For the latter, I scour the normal secondhand bookshops and boot fairs.

Baggins Books in Rochester, Kent has a huge selection of recent and older landrangers and explorers. They may have a web site, a google should bring them up...