An interesting facts from Texas Instrument document about energy harvesting. This is the estimate of how much current can be obtained from different source of energy that is readily available for harvesting from the surrounding environment.
This idea gives a hint so that you don't have to go and research unnecessary thing which provide the most amperage of current. As you can see Vibration and motion as well as temperature difference seems to have more energy to be harvested from.
Why energy harvesting?
It's because the era of curiosity about perpetual energy is about to become reality. In the future many of small devices will be powered by perpetual energy where it does not require any battery and it will last as long as it is not faulty. Imagine a remote control without battery but powered by action when you push its button.