For magnets, I use the D31 magnets from K&J magnetics. These are strong enough that I can hold my tray upside down and the figures won't fall off.
http://www.kjmagnetics.com/proddetail.asp?prod=D31
For sheet metal, I suggest you grab a magnet, go down to Home Depot and browse their sheet metal aisle. I cannot remember what guage metal I used, but it was easy enough to cut with a paper cutter. It is quite thin, so you may want to glue it to a thin wood board to give is some strength.