Originally Posted by creativecrater
I have foamboard sheets that are about 40" by 60". I have no idea where you could buy it because the sheets I have were randomly given to me. In any case, you'd still likely need to use the bricking method to make it strong enough. I got two lengths of my 6 foot boomerang from one of these sheets, but the buster sword is more complicated I think... good luck on your prop though...
I found that foamboard comes in a few standard sizes depending on where you get it from:
From an art supply store: 40x60 inch, 36x48inch, 24x36 inch or 18x24inch
From a home supply store: 4x6FOOT, or 4x8FOOT
Places like home depot and lowes (the home supply stores) will generally allow you to cut your insulation boards down to smaller sizes. You may have to pay a small fee per cut. Art supply stores sell in a wide range of sizes, including sizes not listed. Generally you can't request a larger board to be cut to a smaller size (at least in my general area, all the art supply stores don't allow you to cut the larger boards. They just tell you to get smaller boards).
As for stiffening a large prop made of insulation foam: a balsa board core helps, along with resin. You have to be careful when using resin, adhesives and paint when using insulation foam. Chemical reactions between these products can literally eat the foam away, cause toxic fumes or severely damage your prop if the products aren't used properly. More or less, these products get along just fine when used as directed, and properly applied. Just read all your labels and if you have questions, just ask. There are a ton of people on these forums who are knowledgeable in prop construction.