Anti-particle of the graviton is known as the graviton itself. Like photons. That is the accepted way. To find the anti-particle you have to sum up the quantum values to zero. You might also think that photon is light and there is no anti-light ? Is the same argument valid for gravitons ?
However If we found a material that emits anti-gravitons (Any hypothetical force that would repel masses ), it would be a huge boom in airline industry :) isn't it ? There is a good question. Are there such materials in the center of the earth to maintain the stability against a huge gravitational pull ?
We know that there is exists an equilibrium inside the nucleus (between neutrons and protons ) to maintain the stability under a large electric field. We might observe the same scenario for gravitational fields. Hence we would be able to find materials at the core of our planet which can be used to make flying objects with anti-gravity effect.
Challenge is to geologists. I don't know how deep we have gone through our planet or how complete the knowledge of earth's composition.
However existence of a particle (not necessarily anti-gravitons) that effectively alternate the graviton would be a great news.