The same Pokémon as in Route 124 .
Did you know? You could visit Mossdeep City in the Pokémon Omega Ruby and Alpha Sapphire Special Demo Version, although there was a lot less going on.
When you arrive coming from Route 124, you’ll likely land on a beach with an item ball on it; crack said ball open to find a Big Pearl handily tucked away inside. From there, head to the east and up onto land; you should be able to find the Pokémon Center with ease.
If you go northwest of there while using the Dowsing Machine, you can find a Star Piece . West of there is Steven Stone’s house; while there’s nothing of import there at the moment, you can examine his rock collection to find a variety of stones from across the Pokémon core series’s regions.
Anyhow, east of the Center you’ll also find the Poké Mart. In there, you can speak with the woman at the right and, if asked you want a TM, answer "Yes" to get TM60 (Quash). Head southeast of there after you stock up and along the stone path.
When it passes by a downbound staircase, go down and along the narrow path to the east to find a Net Ball , then return to the path. Continue following it until you go up some stairs, then go northwest and along the edge of the ledges to find a Revive .
In the house north of there, you can get a Super Rod from the Fisherman. After, go outside and north along the path to the launch pad of the rocket; use the Dowsing Machine to find an Iron to the east of the rocket.
At the top of the island, you can enter the Space Center. There, go northwest and speak with the Sailor to get a Sun Stone .
That’s about it. Northeast of the Pokémon Center is the Gym. Go. Now.