There are numerous sites online that sell tadpoles. One is Nature Gift Store, but just Google "live tadpoles" and you will find others. You can buy them live and preserved at Ward's Natural Science Store online, also.
Depending on where you live, you should be able to get them at a local store that sells live fish. If all else fails, however, you can get them through the internet and the tadpoles will be delivered right to your door!