you have to buy some from traders and build planetary core extractor it will need some ceramite to build but its the only way to get ceramite - adamantium. Always save both intill you can build planetary core extractor or if you mined every thing out of the mountains start new game. i think rogue traders are the only ones to have imperial mats and are rare as hell if push comes to shove spawn one in the de bug menu. they are spaces ships.
P.S. i have never played in a non mountains region with this mod

Basically what Major_Lordchris says :)

repair and recycle can also give you ceramite when you recycle armor from raiders.

Ok, now how do you refine ceramite and other materials?

it should be a work order in one of the first benches you unlock. i believe its constuction or materials. i forget the name exactly