I believe there are one or two in the dragon Delv in Western Skyrim
There are flame atronachs in at least two Bolgrul quest delves (Auridon and one of the dwemer ones, either Stonefalls or Alik'r). Also in daily quests in Imperial City and Cyrodiil. Also in City of Ash 1.
And they surely are in some of the towns overrun by daedra in AD territory; I just forget exactly which ones/where. One of them is the first town you get to entering a zone from the Southeast, perhaps Reapers March, but I couldn't easily locate that on a map.
I just got a champion point for crafting clothing. NOT from turning in the daily writ I was doing it for, but just from completing my third endeavor for the day. An add-on said I got 15,400 XP. For comparison, the same add-on subsequently said I was getting 3,292 XP for each writ I turned in.
Then the game crashed, so that's all the data I have.
Where are some convenient supplies of storm atronachs?
One of the three areas around the Spell Scar in Craglorn will have storm (and air) atronachs. It's a group quest area so it also has a fast respawn rate.
That's what I wound up using. It's the north part. Online guides suggest a delve for storm atronachs. I didn't actually find the delve entrance, but I did find a lot of trash packs that included one storm atronach each.
I should add, attack mobs if you see lightning on the ground, one might summon. Also, there's a flame atronach area next door. If you haven't completed your weekly yet either, you can use your class skills on the other mobs and knock three endeavors out in one session.
Meanwhile, what I chose to do for flame atronachs was just to go to City of Ash 1. That isn't a super-efficient way to farm them; trash packs typically have one flame atronach each. But if you don't mind farming generic mobs anyway, for XP or a weekly endeavor or whatever, it's nice and reliable.