Michaels sells thick illustration board in 2x3 ft. sheets, but it's fairly expensive. I work there, so I know. It's great stuff, but it's a bit pricy. Maybe some of the other art supply store would carry something cheaper. The mat board material at Michaels is precut. Unfortunately we don't sell that in sheets, although you might be able to make a deal at the custom framing counter. If you should happen to find anyplace that sells that good thick cardboard, be sure to let us know. I'd love to find a source!
You know, now that I'm thinking about it, maybe a place that sells supplies to architects and things like that would carry it for making models and such. I know there's alot of art stores in Berkeley. I think you should try out there first.