I've heard they are formidable creatures every fighter must have, but also that a ritual must have different kinds of drachs, since they boost eachother. Where do I find each type?

There's a place in Golemus Golemicarum where they can be recruited, but the location is locked at the moment. 

Usually, people receive them as rewards from quests or events. 

You can recruit one species of it in Golemus Golemicarum using a Charm, if you have it. One is available somewhere in the MD Shop, one Christmas species can only be had via trade as far as I know, and one requires a Wishpoint to transform into something that is very rare and powerful.