There are both memory foam and regular foam toppers. You can usually by foam to fit in any density or thickness you lie from either a mattress company, upolster supplier or end of the run fabric place. We have both in Reno so i would think they are not too hard to find either locally or on the net. Search for either "custom mattress" " foam cut to size" or " mill end remnants". Our local pace is actually called MillEnd Fabrics.
I did a quick search for you. You can get foam cut to size on the net from:
http://www.foamforyou.com/Open_Cell_Foam.htmOr in salem where your build is at.
http://www.in-stockfabrics.com/But you should also be able to find an egg crate non memory foam topper for fairly cheap from walmart or target type place.
Then as other posters have suggested, just roll it up and shove it in. With a tear you just need to think outside the box a little!