Fusion plants also need to absorb the neutrons coming out of the reactor and secure themselves a supply of tritium for their reaction. The most common solution is to do all three with a blanket of molten lithium that can absorb neutrons and heat from the reaction, transmute into tritium, and go through a heat exchanger with water to heat it up.
Lithium fission would be into Hydrogen and Helium - definitely not releasing energy. Do you mean that Lithium fusion with a proton or neutron releases energy?
There's a lot of lithium being mined for batteries. Just centrifuge out Li-6 or isolate it with some other clever means. Not much is needed for fusion fuel compared to industrial scale consumers of lithium and it's a healthy boost to heat output.
