The only way I know to get it for free is to make your own. I do it by burning twiggy brush and then dowsing the coals with a watering can. For big fires I shovel the coals into a metal trash can as they accumulate and then dowse.

The contents of a Britta water filter is basically charcoal.
You can ask neighbors to save theirs for you. It's very fine.

You can purchase charcoal in a fish-supply store.