Drugstores:
* CVS
* Walgreens
* Rite Aid
Mass Retailers:
* Walmart
* Target
Grocery Stores:
* Many larger grocery stores with a health and beauty section (Kroger, Safeway, etc.)
Online Retailers:
* Amazon
* Ulta
* Target.com
* Walmart.com
* Drugstore.com
* Maybelline's official website
* And other online beauty retailers
Department Stores:
* Some department stores that carry drugstore-brand makeup might carry it (Kohl's, Macy's)
To find it most easily, you can often use the store locators on the websites of the retailers I've listed or check their inventory online.