I have been asked so many times where I purchase my books from, I will list some of my favourites for ya’ll:
1) Charity shops (usually books are in mint condition, I should know, I worked in a charity bookstore)
2) Skoob Books (http://www.skoob.com/)
3) Black Gull Bookshop (http://www.camdenlockmarket.com/unit.php?id=61)
4) Bloomsbury Book Fair (http://www.bloomsburybookfair.com/)
5) Redhouse (http://www.redhouse.co.uk/) – I am a proud member!
6) The Works (http://www.theworks.co.uk/)