It's simple actually.
In the grass patch right above Solaceon Town, you can find wild Chanseys. When you meet one, try your luck and use Thief on them (do not bother to catch them it's extremely difficult to).
You can get the TM for Thief on the right of the Galactic Eterna Building (cut the tree, then go to the right of the building, and cut another tree). Teach it to a Pokémon, preferably Gligar as it is quite bulky.
Most of the time, Chansey will be holding an Oval Stone and make sure you remove Gligar's item before using Thief too.