Europium does not trade like copper or gold. There's no deep exchange contract you can point at. In practice, pricing tends to be discovered through supplier quotes, contract negotiations, and specialist price reporting.
Two practical markers that show how non-retail this market is:
The U.S. Geological Survey tracks an average price series for europium oxide (99.99% minimum), and explicitly notes the source is the Argus Media price assessment. That's basically the mainstream "reference" for many readers, and it still sits behind a price-reporting methodology rather than an exchange screen.
Policy can introduce "sovereign lead time" (licensing delays and allocation behavior) that moves markets even when physical demand has not changed much.