You don't necessarily NEED a Ralts. Just any pokemon is fine as long as it has teleport, so you can use the abra that you got from trading a bibarel. Also, Ralts is near the train station area that got blown up at the beginning of the game. There's a guy in lower peridot ward where you have to give a bunch of money to and he'll open the way to a Ralts.
Get a pokemon with teleport, go to the pokemon center in Obsidia ward (so you can teleport there), ride a tauros and use teleport (so you return to the obsidia ward poke center), go the Byxbybybyby Wasteland, head to the area where you find mudkip, and there should be a ledge where you can go over with tauros.