I've had success with the foam in my shoes. I had scraps of it laying around (I use it for costuming - had just finished a copy of Kevin Kelm's
robot suit).
Most Joanne Fabrics have it in large rolls - most of the time it's green, but it comes in a variety of colors sometimes. Various thickness also - I use 1-2" thickness, but they make bigger if you have bell toed shoes.