Every episode of Doctor Who I've seen shows him with the Psychic Paper. Does it ever discuss how/where he got it?
The Second Doctor was given psychic paper while employed as an agent by the Time Lords' covert organisation, the Celestial Intervention Agency. The CIA had developed the technology. (PDA: World Game) --Psychic Paper
Psychic paper is a piece of psycho-interactive material that affects the minds of people who see it. The paper makes them believe they are seeing the identification of a person of authority, usually cued by the suggestion prompted by the agent using it. The paper would work on most minds but was unable to affect those of a genius intellect. It has reappeared in the series commonly used by the Ninth, Tenth, and Eleventh incarnations of the Doctor. It can be used as a credit card, an ID for an electronic system (no idea how THAT works) or a keycard. The Doctor has used it to send messages through time, though the results vary widely.
It is one of the two most common hand props of the Doctor. The other is the sonic screwdriver. Psychic paper was seen onscreen in the episode "The End of the World" written by Russell Davies broadcast April 5, 2005.