The sun itself is its own energy source. The fusion reactions create energy, but as the elements get heavier and heavier, less energy is emitted. Depending on the size the star will either cool down, or explode violently (possibly forming a black hole). And probably many fates in-between I don’t know about :)

You can get energy from gravity by moving something closer to a gravity source, but realistically only to a limit. Gravity gets weaker as you get further away, so you can only get so fast from it before either hitting the object, or being too far to notice the pull.