Question

Easy way to obtain a tirtouga fossil in Y?

Answer 1

In Pokémon X and Y a Tirtouga Fossil can be randomly found after smashing rocks in Glittering Cave. Note you have to have defeated the Elite 4 first.

Source: http://bulbapedia.bulbagarden.net/wiki/Glittering_Cave

Hope I helped :)

Answer 2

There's an area inside Glittering Cave where you encountered Team Flare, return to that area and save it. Once you do so just smash the rocks if it's not what you desire just soft reset until you get it.

Source: Experience

Hope I Helped