It seems that finding nice flat foamboard is pretty rare, but I've come up with a nifty solution. Buy a full case of twenty-five sheets, and when you get home take out every other sheet, turn it around, and put it back in the same "slot" it came from. Now the warps are working against each other. When the whole case is done, stuff in one or two more sheets to get it packed nice and tight. After a few days, maybe a week, you'll have a whole case of nice flat foamboard! It worked for me!