The Trading note is an optional artifact in the sewer room with two clickers in The Suburbs. After you knock the pallet into the water, open the nearby door and look beside the mattress at the back of the room.
Ish describes a rare peaceful trade with a group that had scared children with them. He nearly told them about his hideout, and after some doubt decides that surviving alone is pointless, so he plans to find them and invite them in.
This is the moment Ish's one-man hideout starts becoming a community. A later note from Kyle tells the same meeting from the family's side.
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.