Question

How do you get the sail fossil in X and Y?

Answer 1

You can receive it once you defeat Team Flare in Glittering Cave then a scientist will as you to pick between the Sail and Jaw Fossil then go to the Fossil Museum to revive it.

Hope I Helped

Answer 2

You can choose the Sail Fossil or the Jaw Fossil from the fossil assistant in Glittering Cave after defeating Team Flare there.

Source

Answer 3

you can get one by:
when you first go to glittering cave, you ....[skip because I forgot that part]... then you see the professor and ha asks you which fossil you would like. you choose sail fossil, and then you got a sail fossil!

source: memory (what I have of it!)