These two shops have a good range of baking supplies:
Cakes, Cookies and Crafts, based in the UK and carrying an awesome collection of sprinkles (including little gingerbread men and candy canes) and cupcake cases with all kinds of patterns (pictures of ice creams and flowers, and nice spots and stripes).
Imaginative Icing – I visited their shop in York last week and was really impressed. It’s more of a serious cake decorating shop, so they don’t have many sprinkles, but they have all kinds of food colourings and other goods for icing cakes. Most interesting is the edible glitter, which I haven’t tried but my mum has and would, I think, put it on top of everything she eats if she could. They even do holographic glitters that you can eat, but to be honest I don’t think I could – I dread to think what they are made from.