I have compiled a list of sources for the elements that are available to the amateur chemist. Lanthanum will be discussed here.
Lanthanum is a relatively soft and reactive metal of the rare earth metal group. The first member of its group, it forms white or colorless trivalent compounds, making its aqueous chemistry boring. Lanthanum is found with all the other rare earth elements.
In element form: Mischmetal contains about 45% lanthanum, while ferrocerium contains about 30%. Lanthanum alloy is found in the anode of a nickel metal-hydride battery. (Be careful when opening these batteries, risk of fire.)
In compound form: No sources found.
Here is my sample of lanthanum. It is a piece of mischmetal. The presence of rare earths is indicated by the formation of hydrogen and slimy hydroxides when in contact with warm water.