Big box. Probably comes in a 4x8. I got mine from remnants at work on the cheap! You only need a single layer so amount depends on the dims of the freezer and height of the collar. After foaming mine, I applied foil tape.
Lowes carries it 2" Styrofoam in 4x8 sheets. Has foil reflective barrier. Be sure you look at all the options, there are a number. There are also some packs of pre-cut foam in the insulation area of the store.