You can find an alley on 7th Street. there you will find scientists working on a project. after defeating one of them. you have a choice to help them. they will require collecting specific Pokemons for their new creation. after you finish the entire quest, you will battle Type: Null and chose whether or not to capture it. It will be level 50, it's recommended you use a tanky ghost type so it can be immune to its normal type attacks. Because it will use Take-Down and kill itself if you're trying to catch it. Before the capture the scientist gets killed by the Type Null since the pokemon you gave still had memories of you.
Carvana: can be found in Lapis Ward (Post Restoration, Good Rod. Or by using the Good Rod at the Aqua Gang hideout, you also get given one if you join them)
Unfezant: can be found in Ionia Valley (High levels there, so it's easy to go straight to unfezant)
Luxray: can be found in Corner (5000 coin reward, this part is the most annoying, unless you have 100,000 poke dollars on standby to buy game tokens instead)
(it is recommended you breed the Shinx if you're doing it early. since you obtained it from the game corner)
After you gain fly by defeating Ciel, you can return to 7th street and the gate blocking the stairs behind the recovery machine will be open. It will lead you back to the Water Treatment center. It will lead you in the room Taka locked after you decided to battle/not battle him. In the said room is located TM58 Sky Drop and provides a shortcut back.
To the right side of 7 street, there's a dude wanting one of those "Rare candies". He will trade you a Piplup (With moves Icy Wind and Agility) if you give him one. You can find this in the doorway Bennett leaves open behind the mining rock. You will need Blast Powder in order to access it.