Let's start with what works best for getting dimeritium in toussaint since you'll need quite a bit for that grandmaster gear. The simplest way is breaking down dimeritium ingots or those shackles you find on bandits at any blacksmith, and honestly that'll get you pretty far. What's really helpful is crafting it yourself using 1x dark steel ingot and 1x meteorite silver ingot, which you can get from breaking down gear or buying from merchants around beauclair.

While you're exploring toussaint, make sure to check every chest and corpse you come across, especially in those bandit camps and abandoned sites since they often have stuff you can break down. The grandmaster smiths in beauclair sometimes stock dimeritium, but at 436 to 876 crowns per piece it's not exactly cheap so maybe save that as a last resort.

Good to know: don't waste time trying to find raw ore deposits, they're super rare and not worth the effort. You're much better off focusing on dismantling gear and crafting what you need. The materials might seem expensive at first, but if you're smart about picking up and breaking down the right equipment, you'll have enough for that grandmaster gear before you know it.