Acheon

You can get a Trapinch at the Game Corner and evolve it to a Flygon. No others are available yet.

Catching a Wooper that knows (or will learn) earthquake is also an option.

Just wanted to add--grabbing a Pokemon that knows earthquake will also make Aya's gym a whole lot easier. It's a double battle, though, so be sure not to knock out your own team.

Catch a Chingling under the Grand Staircase, then evolve it.

Found it. Thanks!

Mudkip didn't appear for me until it was clear. In regards to the guy on Azurine Island that wants you to show him a Chimecho, where can I find a Chingling? The available Pokemon list shows it as catchable in the wild, but I have no clue where to look.