Crimtane Ore

Mostly from bosses. The Brain of Cthulhu hands it over in bulk and the Eye of Cthulhu chips in on a Crimson world, so treat this as a boss reward rather than a mining target. Demonite clusters inside Corruption chasms, but this does not get the same treatment in Crimson chasms, and digging around down there specifically is wasted effort.

Small veins do generate underground. The ones above sea level come out with a starter pickaxe, while anything deeper needs Gold tier, which most players do not have at that stage. Bombs sidestep the requirement entirely and are the standard early workaround.

A full armor set plus a weapon is the realistic first goal, and one Brain kill will not cover it. Killing it repeatedly is fine since the summoning item is cheap, and the surplus sells well enough that repeat Brain runs double as an early income source.

The ore is inert. It glows and sparkles once placed, but it will not spread the Crimson, so you can store it or build with it anywhere without worrying.