Need Suggestions ASAP For a Cheap Extra Five Point Harness Booster Seat

I am in an emergency need for a new extra occasional use five point harness booster seat for my 4 year old. Our previous nanny is refusing to give us back our extra seat, so I now must find a new seat for our new after school sitter ASAP. My son is 4, about 42 inches or so and not quite 50 lbs. I am not concerned about any special features as long as has five-point harness and is a safe seat since it will only used be three times a week on very short distance from the sitter to our house (less than 4 miles per week).

Thanks in advance for any help you might be able to offer.