Check vendors too, they always seem to have these also.
I've got a bunch of netch leather, while never getting it from netches (specifically the two blacksmith vendors, as well as some other vendors.)
By the way:
I BOUGHT the entire heavy armor stalhrim armor, as well as nordic armor, before I could make either (well, at the time before respeccing, I could not even MAKE nordic.) Then I improved my own nordic armor, even though I can't make it (forgot you could do that... lols) I Bought all that stuff from the smithing vendors too, so I still have lots of stalrhim for whatever (frea is in the other set of heavy stalhrim, looking spiffy.)