This article from Threads #174 (August/September 2014) by Norma Loehr will allow you to begin creating your own foam-cup bras.

Bras are easy to sew and considerably less expensive than ready-to-wear alternatives. An especially versatile type of bra I like to make is one with thin foam cups. Not only can a light foam padding be added to any lovely bra pattern to create a smooth silhouette, it can be covered with almost any fabric that is comfortable against the skin, including silk, lace, or spandex, without having to refit the bra or sacrifice support. This approach to bra-making enables you to customize bras for the look and fit you want.