GameStop offers the Star Wars BB-8 USB Car Charger for $29.99. Opt for in-store pickup to dodge the $4.99 shipping fee. That's the lowest price we could find by $10, outside of the mention below. Measuring 6" x 4" x 4", it features two USB charging ports, a moving head that rotates and makes sounds, three volume levels, and 36" cable.
Prefer not to go to the store? ThinkGeek has it for $24.99 plus $6.95 for shipping.