You can buy natural charcoal at stores like Lowe's, Home Depot, Walmart. Though it may be a seasonal item, and you would have the most luck in spring to late summer.

You can also find it at some garden nurseries. At the nurseries you will find smaller pieces for mixing in the soil. The others are for grilling, but are not treated with fire starter.

There is even some made by Kingsford, just make sure it is natural hardwood charcoal. It usually runs about $8 for a bag of about 8 pounds. (I am guessing at the bag size.)