One of my favourite sources for small "puffy" stickers is, unbelievably, Assessorize, the clothes/accessories shop. I stumbled upon them on a stand in the children's area and have been a huge fan ever since. They are small and ideal for making toppers.
They are quite cheap, about £1.75 per pack, and I can usually get five to six toppers from one pack. They have several different sets at various times of the year, but be warned, get them when you see them, because once they are gone, they are gone!
My sister is on "special watch" for their Christmas stickers. Last year they were fabulous, (see Sizzix Christmas Matchbook), cute santas, reindeer, penguins, trees. I can't wait to see what they come up with this year. The cards I made with them just flew!Paperchase, the stationary shop, also do brilliant stickers, about the same size as Assessorize but tend to be a bit cheaper. The cake topper above is made using Paperchase stickers, but I haven't got any pictures of their packets. Again they are seasonal and well worth looking for.