I'm certainly no expert in the matter but couldn't this be constrained in other dimensions rather than availability of the source material. For example capital

I'm certainly no expert in the matter but couldn't this be constrained in other dimensions rather than availability of the source material. For example capital requirements of whatever extraction / purification steps are required vs the global demand. Therefore increasing demand could cause investment which increases supply? That said, if Fusion power ever becomes a reality I hope it doesn't depend on some difficult to source material.