The Prune Danish via eBay offers the Key Single USB Car Charger, model no. PCUM20004GLK, for $2.95 with free shipping. That's $7 off list and the lowest total price we could find. It's universally compatible.

Also available is the Key Double USB Car Charger, model no. PCUU40008GLK, for $3.09 with free shipping. That's the lowest total price we could find by $2.