Pokemon can also randomly return with any of these Evolution Stones from a Pokejob. Access Pokejobs from the terminal in any Pokémon Center. You can also find stones at these locations.

If I have, at best, a 1-in-300 shot at getting a single evolution item, plus a 1-in-5 chance it's the right item, you can see how those calculations start to spiral out of control. (Photo: Niantic) ...