Erbium is cheap enough.
As is Hafnium.
There is a surprising amount of these exotic materials availlable cheaply since they have very few uses in what might be called 'domestic markets'.. I recently bought some zirconium foil in Ebay ex 'Goodfellow' metal suppliers in the UK, still in it's original packaging. I paid around £5 for it, Goodfellow charge around £300.