They apparently show up as random loot in chests. I have run across several in loot chests.
Why would you sell it? I think the game makes it pretty clear that you need one. I think when I talked to Glover he explained how you need it to mine Stalhrim.
I accidentally sold mine and got around that by saving in front of Fort Frostmoth and going through the dungeon until the loot chest behind the master locked grating, in the room with all the Eidar cheese wheels and ale.
If the chest didnt contain an Ancient Nordic Pickaxe I would reload.
Took me about a dozen times, but i finally have the pickaxe back
