[DennisP]

This article by a Berkeley physicist does the math on fusion fuel: https://dothemath.ucsd.edu/2012/01/nuclear-fusion/

Deuterium fuel is the most abundant. There's enough in your morning shower to supply all your energy needs for a year. There's enough in the oceans to last for billions of years. Fusion is as close to renewable as anything, because it'll last until the sun goes out.

Right now most projects are also using tritium fuel, which has to be made from lithium. That's plenty abundant but not to the extreme of deuterium. But pure deuterium fusion is possible, just a little harder. And one prominent fusion startup, Helion, is actually using deuterium (along with helium-3, which is the waste product of deuterium fusion).