Note to Brother is a handwritten letter on a table upstairs in the spore-filled building of Beyond the Wall. After the first infected fight, climb the stairs, turn left before the hole in the wall and open the closed door; the note sits in the next room.
Mark explains that his group was waiting outside the wall for a smuggler to sneak them into the zone. A patrol came by, they hid in this building, and nobody noticed the spores until too late. He hopes the smuggler will pass the note on.
Mark and his friends are already infected when he writes it. The runners you meet in this building are almost certainly his group, and the injured smuggler with the broken gas mask you pass just before is the contact they were waiting for. The note is still lying where he left it, so his brother never got it.
Boston Q.Z. Map
The Boston Q.Z. Map is the first artifact in the game and you cannot miss it: it is already inside Joel's backpack when you pick the bag up off a table in the smuggling tunnel during Beyond the Wall.
Docks Note
The Docks Note is a gripe from one of Robert's smugglers, lying in the big warehouse at Mission Hill Pier where a fishing boat hangs over the loading bay. Look on the right side, against the railing behind a blue trash bin.
Drafting Notice
The Drafting Notice is a FEDRA work order lying in a heap of papers in the Boston slums. After the bus where Malick confronts Joel, walk straight on between a vendor's stall on the left and a canopy on the right; the notice is on the ground at the corner.
Shipping Manifest
The Shipping Manifest is a sailor's note at the Mission Hill Pier docks. After you vault the wall down among Robert's patrolling men, go right to the railing above the water; it lies by a fishing net behind a blue trash bin.