Homemade Chalk Recipe

Here is one from right here at Thrifty Fun: http://www.thriftyfun.com/tf494134.tip.html
'Plaster of Paris' is available at craft stores, hardware stores, Kmart etc.

Favecrafts.com has directions for making homemade chalk.

For sidewalk chalk, try about.com, for kids chalk just google, "homemade chalk recipe"