Amazon offers the Cra-Z-Art White Chalk 16-Count for 99 cents with free shipping. (Staples charges the same via pickup. Non-members can spend $49 to qualify for free shipping; otherwise, shipping costs $6.18.) That's $3 below list and the lowest total price we could find. This school-quality chalk is non-toxic.
Update: The item at Amazon is no longer available; however, Staples still offers it for 99 cents with in-store pickup.