You can buy trace mineral drops, which contain a fair amount of lithium, as well as magnesium and other trace minerals. The most common ones are based on de-salted mineral deposits from the great salt lake.

No idea if this is the best supplement for lithium (but would be interested to know).

I have been using these for years, primarily because I feel the evidence is strong for the benefits of a small amount of lithium. As a welcome side effect, the magnesium has completely alleviated leg jitters while sleeping (this is a well known effect of magnesium).