they are a thargoid product
to find them in the market with no help will take forever because they are only sold in a couple places
Maia sells them
in the systems directly around maia you do not have to find a crashed ship to find barnacles
but since the discovered crashed thargoid ships came into the game, they are the easiest fastest way to find barnacles.
you can harvest them from the barnacles that produce them, in many places, but Maia area is the best place to start
if you look for any crashed thargoid ships in Maia area you will find barnacles growing around the thargoid ships, they have pods growing from them, the pods contain the meta alloy, not always, as any rock can do, so can a barnacle give other materials too, but it is where meta alloy is from
the barnacle is a thargoid mining construct it is alive, it refines materials from the ground and sends the refined product into the pods it grows.
shoot the pod and eventually you will get some meta alloy.