If you haven't blown up Megaton there are the travelling merchants that come by.

Kill them and their guards (the guards are harder than you think) and search their bodies. They have the schematics on them.

Also, take their keys and they have quite a bit of caps on their brahmin.

If you head back into Megaton and go back out, right away another will be waiting until you have killed them all and no more come.

If you did blow up Megaton already you can do this with the merchants at the base of Tenpenny Tower.