As has been said, permanent magical tattoos are in the core rules, as slotless magic items. I think the reason that few people pick up on that is that you can't usefully find tattoos as treasure; and if you're crafting your own items, you'd tend to go for a slotted item as it's half the cost.
Which gives me an idea for a new item: a set of magical tattoo inks, that can be used to create a specific magical tattoo. The tattoo artist need not be the caster; rather, it takes 4 hours and a DC 15 craft (tattoo) check to successfully create the tattoo, which must match a picture drawn by the caster.
The cost and price of the ink is the same as for a slotless item of that type (for example, a tattoo ink set of armor +4 (as bracers) has a price of 32k gp).
So, maybe the caster lives in one city, and ships the inks to another city where the buyers are and an expert tattoo artist lives. And sometimes, the shipments are intercepted.