Yokai don't need a source of mana, and can basically be anything classed as a yokai in Japanese mythology. As long as it's not too powerful, and has an excuse to be around in human form(unless there's a very good reason for it to be around in non-human form). As for a familiar, the ideal source of mana is a mage(onmyouji have a different method of creating magical servants), who supports them with both mana and reinforces their existence(which I should probably add a bit about in their description since it's kind of important and all). Basically familiars are created by an infusion of mana, and the mage doing the creating "convincing" themselves that the familiar exists. Familiars need this, or else they become unstable, if they manage to survive losing this.

Therefore, a familiar living off of a mana source(such as an artifact with mana, or maybe even managing to scrounge together ambient mana in the area) separate from a mage will still be unstable and often sick.

Um, does that help? ^^;