Actually, when the Earth was formed, it contained a non-negligible quantity of plutonium (supernovae produce all the elements until Z=100, much over the Z=94 of plutonium, but the heavier elements had already decayed before the interstellar dust reached the nascent Solar System).

Because the longest-lived isotope of plutonium has a half-life close to 100 million years, so some 45 halvings have happened since the Earth formation, almost all primordial plutonium has decayed by now.

A few atoms of the primordial plutonium are still around us, though they are much too few to be easily detectable (there have been claims that some very sensitive experiments have detected them).

The working fusion reactors will produce enough to not need additional tritium, it is a self-sustaining reaction after all.

Tritium has a half-life of about 12 years, meaning that it essentially doesn’t exist naturally. It’s all man made.