Hey does anybody know how to get powdered mammoth tusks? Does it have anything to do with regular mammoth tusks? Or is it a nearly impossible find?

apparently its super rare. wiki says

One is found inside your home in http://elderscrolls.wikia.com/wiki/Markarth
Some may be found at the giants camp http://elderscrolls.wikia.com/wiki/Stonehill_Bluff just north of http://elderscrolls.wikia.com/wiki/Whiterun in a stone bowl next to the bonfire.
Near the beginning of the first cave section of http://elderscrolls.wikia.com/wiki/Mzulft, in an http://elderscrolls.wikia.com/wiki/Apothecary%27s_Satchel on a raised wooden platform by a dead Synod Researcher.

You cant make it from mammoth tusk, you have to find it.

I found a few from the forsworn

Ok Thanks. The fortify Sneak is what I'm really interested in.

Youd be better off using other ingredients with the same effect - the mammoth tusk seems quite rare.

i only came across this 2 , can't recall from where. probably either around Solitude or Markath.

you always can get similar effect from other ingredients. this is nothing special or worth the effort to farm it.

from skyrim wiki:
The following alchemy ingredients can be used to create a potion of Fortify Sneak:
Abecean Longfin (2)
Beehive Husk (3)
Frost Mirriam (2)
Hawk Feathers (4)
Human Flesh (4)
Powdered Mammoth Tusk (2)
Purple Mountain Flower (2)

you would be best off with the purple mountain flowers since there are tons of them.