I've never seen those things for sale at all. I don't think that's something anyone would have extras of to part with; i'm short a few myself.
I'd suggest going to an office supply or maybe a home improvement store and see if they have pieces or sheets that can be cut to the right size. You'd want to start with the right thickness since styrofoam wouldn't cut that way very well. Or you could save a piece of packaging from a DVD player or something (or maybe even go to Wal-Mart or Best Buy and ask if they've got some they're throwing out! I used to work at Office Depot, and we saved boxes and stuff for people all the time).
You won't end up with a perfect replacement, so if you insist on having original stuff you'll have to keep looking. But it should make a reasonable substitute if you just want something in there as a spacer.