buying one is the easiest, but making one will charge it with far more energy and i'd recommend it myself. it should be made of wood; maple, mahogany, purpleheart and poplar are some good choices.
take the wood and make a rectangle with one piece. sand it down, you can paint or carve or burn the alphabet, 0-9, yes, no, and goodbye onto it. if you want to add some decorations [like if you study faerie magic, or celtic] you can do that as well. once that's all done, put a couple layers of lacquer to make it shiny and smooth.
the planchette can be anything really, and you can make it from many types of things in different shapes. you can even use a penny or a cup. if you want to make a typical one, carve a piece of wood into a triangle. you can carve a design into it as well, and add a layer of lacquer. on the bottom of the planchette, i'd take 3 round glass rocks [can be found at an animal supply store in the fish isle] and with a hot glue gun, glue one at each point to help it slide.
you can make it with cardboard if you want, but wood would be best. hope that helped. why are you making one?