Where to buy die-cut foam for battery trays?

[R00ST]

A buddy and I are trying to mock up some battery trays out of steel until mike gets the twin force trays in stock. we now have the bare metal trays, but I was hoping to line them with that thin die-cut foam to help cushion and prevent shorts.

Where can I find this stuff? So far nobody even knows what im talking about at the hardware stores lowes/menards etc... (is there another name for the stuff?)

Any help is appreciated! I was hoping to not have to just use electrical tape!