Patent Brokers?

Does anyone know of any good patent brokers, particularly any who are willing to work for a seller (patent owner) on a pure contingency (success) fee basis? I am looking for such a service to assist clients who have obtained patents, or who have patent applications pending, but who have either been unable to commercialize the invention or who have run out of funds to pursue their patent or application. Such people often have created valuable technology, if only they could find a buyer or licensee.

Of course the difficulty is that if they have run out of funds, they cannot afford to pay a broker to conduct a search for a licensee or buyer.

Any help that anyone could provide would be greatly appreciated.