Things that boost different bender skills We know that the moon boosts waterbenders and the sun and the comet boost firebenders.

Are there other things that boost earthbenders and airbenders?
Such powers are not been mentioned in the Comics or in Avatar or Korra Series. That said, the powers from the Sun and the moon can be seen in the negative sense. Because Night leaves the Firebenders vulnerable a lot (the reason why General Iroh asked Admiral Zhao to wait until daybreak for attack again at Northern Tribe in Episode 19) as WaterBenders becomes virtually unstoppable under moon. Similarly, WaterBenders lose a lot of their power to Sun and they become vulnerable (that's why Admiral Zhao continued the attack only in Day). The powers are more like in Cycle, one is bound to overpower another. Earth and Air on the other hand are elements that have no relation with any natural phenomenon in Avatar World, thereby, making them virtually impossible to be boosted unless by Physical and Mental Training. Besides, Air seems to be the only element that is present everywhere, whether a wooden cage, a metal cage, a ship to hold Criminals, or any other ordinary or extraordinary place. This makes Air as a gift to be virtually available  everywhere for any Airbender. Even the Fire has its limits (remember the way Pa'li was caged before being freed) on availability.