Question

How can you get a Skull Fossil in white?

Answer 1

You find it in twist mountain after completing the Pokemon league. A man in there will give you a random fossil. It is a 1 in 7 chance and you can get it again each day. To find him you go in through iccirus city and take a left. Then you walk through the door. You will be outside and then you make a right until you see a door and then you enter it and he will be in there.
Source: experience

Answer 2

>In Black and White and Black 2 and White 2, a Worker in the Ice Rock room of Twist Mountain hands out a random Fossil from previous generations, including the Skull and Armor Fossils, once per day. If brought to the Nacrene Museum, the Skull and Armor Fossils can be regenerated into Cranidos and Shieldon, respectively.

Source: Bulbapedia/ Skull & Armor Fossils