Praseodymium and Samarium drop like candy in both High metal and metallic mining locations if you do a spot of mining for 15 minutes.
Like literally, you'll walk away with over a dozen units of both per mining trip no matter what you mine due to how mining and materials and Engineer commodities have been applied to in global fixed amounts so no matter which you mine you'll always walk away with silly amounts of them.
Same with Osmium.
In 2 days of mining a short while back, think I sold like 40+ units of osmium, 30+ units of Praseodymium and 30+ units of Samarium, and still had 4 units of each in cargo for Engineer use and that's very conservative figures whilst getting so sick of finding all 3 that I started trying to avoid them. lol