An easy way to get free foam is to us computer packing foam. It comes in all kinds of neat shapes and in either grey or white. Depending upon the brand of the machine, it can be really soft like pillow foam, or more like regular styrofoam. You will be able to cut the foam to shape, wrap with ducttape to hold it all together and then paint it whatever color you need.
Companies get new computers fairly often and they just toss out the packing foam that the machines are packed with, so ask around at work or school (or have your parents / friends check at their work) so you can have them save it for you. I'm sure between now and the convention someone you know will be able to obtain some of the foam.