Press ~ to bring up console.
Type [help stalhrim 4] and enter.
There are a bunch of item codes shown, whick should all start with the same two digits. Remember these.

Now type [player.additem xx02b06b 1] in which you replace the xx with the two digits you were rembering. Press enter. There should be one Stalhrim added to your inventory. You can change the last ‘1’ to any quantity you like, in case you need more Stalhrim.

Hope it helps!