If you discover additiona locations, please add them to the list. Include as many details as you can and a map if possible.
Stahlrim deposit can be found to the Northeast of the Northshore Landing
see map below for exact location ("X" marks the spot)
Alse one on the SE from the Stalhrim Source Map:
On a tiny island at the northeast shore of Solstheim
Inside an area
There is door with master lock you need to pick, but perhaps there is key somewhere. The key can be found by locating all of the Deathbrand treasure chests.
Raven Rock Mine
There are 3 pieces of Stahlrim to get in the Mine near Raven Rock