craftgrrl | halloween costume help

Hello crafty ladies, I have a bit of a question regarding where to buy foam.

Basically I want to go as ms. pacman for halloween, and I want to take foam (egg carton foam, carpet foam, etc.) and make a circular shape. I would be using a hula hoop to give it shape, and duct taping the foam together.

The question is...where would I find foam? I had the idea of home depot but I'm not terribly positive. Any ideas?

EDIT: Thanks ladies I'm off to the fabric store per the advice!

And so it's not text only, here's a lunch box I covered for a British themed swap