Richard James Weldon are the company most of the British tailors use. I'm not sure if they deal retail though. Give them a ring.

http://www.richardjamesweldon.com/

Here's a place in NYC I've read about but can't remember where:

http://tenderbuttons-nyc.com/

Here's one in York in Britain.

http://www.duttonsforbuttons.co.uk/