Fawkes is located in Vault 87, where you have to get the G.E.C.K.
The Museum of History is where a quest involving Abraham Lincoln takes place (you get the Lincoln's Repeater, a unique Lever-Action Rifle [as of Point Lookout] that uses .44 Magnum rounds as part of it).
Strangely enough, You don't repair Lincolin's Repeater with other Lever-Action Rifles, but with regular Hunting Rifles easily found all around the Capitol Wasteland.
After you get out of Raven Rock, you will meet up with Fawkes outside, he will then offer his services to follow you and aid you. However if your karma is to low he will not meet you there. If that’s the case he should be inside the entrance to Underworld aka the Musium of History. He also goes there after you tell him you no longer need him to follow you and you can then ask him to accompany you again.
There is always the possibility that he got killed, he is tough but not indestructible, or simply disappeared from the game. (characters may glitch and disappear, in which case reloading an earlier save is the only way to get them back without the use of console commands on the PC version) ReapTheChaos 21:54, January 29, 2010 (UTC)