If you want to get original patches you should definitely try finding them on Ebay. They are all over. Look under collectibles - militaria - world war II.
If you want to get reproduction patches you may want to try At the Front, What Price Glory or World War II Impressions (look them up on Yahoo). They are dealers that specify in reproduction World War II uniforms and other items for re-enactors. They are all making very good quality reproductions that are extremely close to the originals.