You local market will probably have a haberdashery stall, or you might have a haberdashery in your local town centre. If not try John Lewis.
Rideaway stock full sets but not individual buttons.
If your buttons are fairly plain you will be able to get a replacement fairly easily at a haberdasher, just take one of the existing buttons in and ask them to match it. If they are fancy you may have to change the lot so that they all match, still take a button in though as you need to get the right size.