I have Feed Me I'm Yours by Vicki Lansky and there are recipes in there for everything! There's 5 recipes for playdough, but I think this one would work.
1 C cornstarch
2 C baking soda
1 1/4 C cold water
Mix cornstarch, baking soda, and water. Stir in a saucepan over medium heat for about 4 min or until the mixture thickens to the consistency of moist mached potatoes. Remove from heat, turn out onto a plate, and cover w/ a damp cloth until cool. Knead as you would bread dough. Shape as desired or store in airtight container or plastic bag.
To color, add a few drops of food coloring to the water before mixing it with starch and soda. Or, objects may be left to dry, and then painted w/ tempera or acrylics. Dip in shellac or brush w/ clear nail polish to seal.