Hello everyone!
I was flipping through my Dick Blick art supply catalogue recently and found two products that might be of use to some of you.
Blank playing cards. These are available here. They come in two sizes and are pretty inexpensive. While the designs on the back leave something to be desired for my purposes, they might be game appropriate for you or might be able to serve as prototypes.
Magnetic spray paint. This is available here. The product may be misnamed, however. I am not sure whether it is technically magnetic, or whether it is a non-magnetic metallic that magnets will stick to (The webpage says this "creates a surface magnets will stick to."). At any rate, it might be a good product to look into for making game boards for magnetic pieces.
Hopefully this is helpful!
By the way, DB will ship you their catalogue for free. It is fat and filled with full color photos of the products. I have found it pretty entertaining just to flip through... It can be ordered here.
