52 Cupcakes (linked above) introduced me to a neat website, Confectionery House, which carries a wide variety of cupcake liners. My children are going to have fun picking out some new designs to use in future months.
Last weekend I placed an order for baking supplies with Bakers Catalogue -- including shamrock cupcake liners on sale to use next March :).
I also bought Cup-a-Cake containers, which I was first introduced to by Mrs. Happy Housewife.