Amazon offers the 11-oz. Liquid Wrench Penetrating Oil Spray, model no. L112, for $2.89. Check out via Subscribe & Save to drop it to $2.46. With free shipping, that's the lowest shipped price we could find by $5. It purports to loosen rusted bolts in just a few minutes.
Be sure to cancel your subscription after receiving your item to avoid being billed for subsequent shipments.