It seems as though the Button Box has gone even though it is still listed on the Internet. The Bead Shop in Nealís Yard doesnít sell buttons.
There is a button shop at 76 Marylebone Lane. You can check out their website.
Liberty sells buttons. They have a website too.
I really think you will get a much bigger selection on the Internet though.
Have you looked on Ebay? There are loads.
I know it doesnít help you at the moment because the next show in London is in October but have you ever been to the Knitting and Stitching Show at Alexandra Palace? It is a really good place to stock up!