I picked up several variations of these types of storage containers from my local hardware store (for "House on the Hill"). I'm sure most crafts places would also have them. They're really handy.
Wal-Mart has em in two departments. Fishing tackle area and where they sell fabrics. I have since modified all of Descent, Tide of Iron w/ expansion and Memoir 44 and ALL of it's expansions with these storage boxes.
I actually received a bass pro shop gift card for $100 and spent a good amount of it on these storage boxes for my games. Everyone wonders why I have fish imprinted on the boxes that organize all of my game pieces.