Fallout 3

It's in the museum of history offices hanging behind a desk somewhere. Up some stairs just after finding a john wilkes booth poster and a civil war draft poster.

You better shoot feral ghouls and radiated ghoul first before obtain the poster and there are more arfetacts around the building.

I had ALOT of trouble on this one too.

It's on the second floor on the WALL above the desk.

Just follow the pipboy to the dot on the map then look on the wall.

It will only be available after you start the "Head of state" quest, It will be on the desk on the right hand side when you come up the stairs to find the "Linclon's voice" artifact. It will be on the wall behind the desk. I can't remember the name of the area but its in the Museum of History and when you go in to the atrium of some sort (A big open space) you will see dinosaur's bones and ruined monuments. Make a left its the first door to your left. Then when you head in go up the stairs and head towards the far back left corner. Go in that door and just make your way towards gun sounds. You will see a turret on the sealing. If there are stairs on your left or right head up the stairs, Then right in front of you should be a desk. On the wall behind the desk there SHOULD be the Linclon Memorial poster. Hope this helps!
- Six Pax