CoragonJames HussarCoragonRebecca HailPraesodymium is mining only afaik
That or getting it as reward by completing missions.
I've found it at surface mining locations and in crashed canisters on the surface also, but for sources least prone to dice rolls, mining. It's still a dice roll, but you're more likely to find it in asteroids.
Try Bhare A 3 Ring, I'm running into Praseodymium and Samarium so regularly there that you could say it's common. Followed by Osmium, Palladium, Platinum and Painite. The Ice ring of the same asteroid belt should be visited as well.
In that very ice ring, I scored 6+ tons of low temp diamonds, along with a slew of the usual ice stuff, in one trip.