Here are the basic steps in creating one for yourself.
I buy a sheet of galvanized steel from Lowes (be sure to take a magnet with you) I use tin snips to cut it and then I add about 5 coats of chalkboard paint to it.
Cut a piece of cardboard to the same size as the galvanized steel and add it to the back for extra support.
I added a few magnets made from vintage buttons and a piece of chalk that I found in box at an estate sale. I hung the chalk from ribbon tied to a little eye hook that I attached to the top of the frame.