How do I get powdered mammoth tusk?

Solution 1:

There is currently no way to break down mammoth tusks to get powdered tusks. The only way to obtain some is to purchase it in an alchemy store or to find it as loot. Source

Or if you are desperate for some and are on the PC, you can do it via the console:

player.additem 6bc10 [amount you want]

Solution 2:

You can actually find some, Somewhat rarely in the camps of the giants and their mammoths. There was a quest where you had to get some by going to a giants camp, and by going to other camps I have come across some, but not much, the other way as described above is buying it at alchemist shops but they only sell 1-2 at a time and that is too quite a rare buy.

Good luck with it.

Rob