My go to method of choice to start with dedicated magic playthrough is to generate the world to 300-350 years, start new character and check families in all Tier 3 dwarven towns if any family has build Geomantic Halls. It generally works with all magic skills (floromancy and pyromancy as well).

You can kind of preview town buildings by just going through family list, and check for geomancy halls building sprite - a magic-looking purplish book. If there is none, just make another world, usually 2-3 tries are enough.

You can rarely find Geomancy skill book starting from troublesome encounters, and craft it once you hit lvl 44 in geomancy - similar to other vanilla skills.

If you want to start with/cheat having higher potency, you can use console command (they are available in modding docs within the game folder, the skill id is simply geomancy) which can give you training in any skill.

There is also a mod on workshop that introduce dedicated bookwriting as a skill, which allows you to craft paper/write books for skills - and there is a compability patch for Geomancy for it.

Thanks for this, I often have trouble finding the trainers for geomancy, but I love the skill. Could there be a mod to increase the potential of the skill, or an expensive skill book we could craft?