Hello Heather
Have you been to the wonderfully named Clerkenwell Screws which is in London at the beginning of Clerkenwell Road, (Theobalds Road end) which is a small shop specialising in only one thing. No website but here's a link. They have minimum quantities but are never too expensive and their stock is huge.
http://www.allinlondon.co.uk/directory/1100/15422.php

All the best
Mike