Also, if you use Rock Smash in the Ruins of Alph, you can find the Helix Fossil, Dome Fossil and also Old Amber. You can also find Heart Scales and other hard to find items like Max Revives.
All fossils can be converted into Pokemon in Pewter City at the museum. The guy will be located in the far right of the museum.
Then, you will have them on your team vs looking at them in the Pokedex!