When do you receive the fossil in Black/White?

Question

Because I'm really hoping I can get an Archen so it could be my "Flier" in the game, so it would better if I get it before the Fly HM.

Answer 1

The Fossil Can be Obtained in the Relic Castle of Desert Resort, prior to the Fourth Gym. On the first floor of the castle from it's main entrance (the one with the darmanitan statues outside it). A Backpacker will give the option of cover or plume fossil. In your case, choose plume, and head back to Nacrene City, where your plume fossil may be revived into a level 20 Archen at the Nacrene City Museum

Source: Experience