No doubt Amazon is the best place for wall decals.

From the original answer:

There are many stores that sell large wall decals. Lowes, Home Depot, Walmart, and Target sell wall decals. Arts and craft stores such as A.C. Moore and Hobby Lobby also large wall decals.

One can purchase wall art decals by visiting the popular auction website eBay. Other websites that offer these wall decals for purchase are AllPosters and Amazon.

Custom wall decals can be purchased from several online retailers. Dali Decals is an online retailer that allows you to see examples of custom wall decals on nursery walls to help you envision what the final product will look like.

From the original answer:
It really depends on what kind of ambiance is looking to be created for wall decals.