Lithium is a super common element. They are mining tens of thousands of tons per year increasing rapidly as make electric car batteries. Lithium-6 is 5% of lithium. Fusion reactors only need tons for breeding blanket.
The original ARC design had 950 tonnes of FLiBe, which contains 115 tonnes of Li-6. This is a substantial fraction of the Li-6 produced for the entire US hydrogen bomb program, all for a single 190 MW(e) (net) reactor.
Sure, the input feed is not the problem. The problem is actually separating it. The technology used for the H-bombs is no longer acceptable due to mercury pollution. World Li isotope separation capacity currently is in kilograms per year (to produce pure Li-7 for pH control in fission reactors.)
And then there's the beryllium. A single one of those ARC reactors would use something like 40% of the current world annual beryllium production.