You have probably got lots of builders merchants near you, like Wickes, B&Q, etc. Try one of these or look up local builders merchants on yell.com. Good luck ! Are you near P J Scott at Kempton Park? If not, ring them and they might tell you of someone near you. (Think they might have changed their name recently, also try Budd & Bird)
Any Builders Merchants sells them, but if you are after specific bricks to match your existing then you either need to know what they were (and back to Builders merchant) or go to a so-called brick Library (use Yell Pages or ask at Builders merchants). These organisations charge quite a large margin for finding you a matching brick.