The Sewers note is an optional artifact in the storage room Ellie opens by crawling through a vent in The Suburbs. Once she unlocks the door, head to the far left corner and take it from the table.
Back on land after months away, Ish finds both infected and survivors trying to kill him. He decides the sewers are a good place to hide, since their many tunnels make them easy to defend and easy to vanish into, and jokes about becoming a sewer mole-man.
The second Ish note. It explains why the tunnels you are crossing were turned into a home.
Antiparticles
Antiparticles, issue 11 of Savage Starlight, lies on the console in the bridge of the wrecked fishing boat on the beach at the start of The Suburbs.
Boat note
The Boat note is an optional artifact on the bridge of the beached fishing boat at the very start of The Suburbs. Climb onto the deck, step into the wheelhouse and grab it from the console beside the Antiparticles comic.
Bomb: Containment
Bomb: Containment is a training manual on a shelf in the sewers of The Suburbs. It makes nail bomb blasts 1.5 times wider.
Cornered note
The Cornered note is an optional artifact behind a door near the sewer ambush site in The Suburbs. After the runner and clicker attack, climb the stairs, go through the door on the left and find it on a bench by the bodies in the corner.