Because of how entirely permanent this is, it pays to look far - like millions of years. That's what it took to accumulate.

We know how it came to exist naturally it’s really just the matter of someone being crazy enough to try and replicate those underground conditions and spend the money on the project.

How that works is just normal radioactive decay. The conditions aren't actually important.

It's probable we'll be able to make a bit with fusion, but the amounts will be small. IIRC we also collect some from decaying radioactive things in manmade settings, but again, it's hard to beat an chunk entire planet underneath a salt dome. Next options are harvesting it from space (but it's really spread out) or a gas giant (but there's stupid amounts of gravity).