Lucky Electronics via Amazon offers this 12- to 24-volt Triple-Socket Car Charger
in Black for $3.30
with free shipping
. Outside of the price below, that's $2 under the lowest total price we could find for such an adapter. It converts a single 12-volt car cigarette lighter outlet into three.
Of note, this item ships from Hong Kong and may take several weeks to arrive.
A close price: eforcity via eBay charges $3.84 with free shipping.