Any vet zone afaik, but a rare drop is a rare drop. I have 70+ days /played and I loot every furniture item and I've never found ancient elf. I found all the other rares once, daedric twice.
Until Orsinium, the only place to loot Ancient Elf motifs was in the Cadwell's Gold zones. Now, all 4 purple motifs (Ancient Elf, Daedric, Barbaric, and Primal) all have a chance of dropping in Orsinium. So, low-level characters should be able to loot them - however, they still won't be able to learn them until they have the appropriate material level unlocked in one of the equipment crafting lines.