In the sewer, near a couple of skeletons. The section of sewer containing the note and the supplies is entered from either the manhole in the alley east of Seward Square North Metro, or the manhole northwest of Metro Central on Pennsylvania Avenue. The tunnels are patrolled by a few leveled super mutants.
Behind a door in the south part of the local map is a metal stairway leading to the skeletons, where you will find the holotape and a laser pistol. As the note indicates, at the end of the southeast tunnel you will find several ammunition, mine, and grenade boxes, along with a Tumblers Today, in a hole in the wall.
JP is a lying, toilet-sucking, good for nothing, rad-brained bastard.
The stash doesn't exist. We searched everywhere down here and nothing, not one lousy gun. We managed to pick up some ammo, but that's about it. Now we're stuck here with these damn mutants streaming in and out of the tunnels. What are those bastards even doing?
I snuck some of our supplies at the end of the southeast tunnel under some debris. There's no way we can sneak past the bastards carrying it all by ourselves. Hell, I'll send JP down here next time to retrieve it - see how he likes it.