I have found that in my PS3 version of Fallout 3 that Griffin (Ghoul) has an unlimited supply of caps on him. If you kill him and put his body in a safe place (You can find him at the history museum after doing a few quests for the locals) he will have up to nine-hundred caps on him every 4-5 In-game days. Although this may just be a glitch in my game, it may work for all consoles.

                                      - TheCanadianMau5.