Someplace that you can go and try lots of different ones to see which one fits you best.
Then return home and order it online cheaper
( M o D, Mon 1 Jun 2009, 13:58, archived)

This is the plan
but I don't know any other shops, or where there might be a concentration of that sort of thing.
edit: Though badger says Covent Garden. Righty-oh.
(rnuk http://flickr.com/rnuk, Mon 1 Jun 2009, 14:01, archived)