Reveal all my sources? That'll be the day.
Flea markets seem to be more of a US thing, though I have been to them all over the place. There are some markets, what you mainly want to look for in those is those that do house clearance/estate sales. After that I do OK from a lot of the pawn shops around here, some are staffed by people that know games but not all. The humble car boot sale is where a lot of the action is at, if there is ever a road/area sale then definitely do those and maybe also consider computer faires/fairs/fayres.
It has been years since I did London proper for this sort of thing but I did have some good stuff from the people that set up under the shadow of westway around all the skateparks (we mainly saw them on the run up from the underground to the playstation/bay 66 skatepark). That I still call it playstation probably says how long it has been though.
If you are feeling especially sociable then have a walk around places and speak to residents. Many will be quite happy to make £30 if they are otherwise standing around that day -- "I struck out at the nearby market [gesture with thumb], however I thought I would take a wander and ask". However in those cases I would not be picky unless you are literally looking at 10 copies of fifa and wrestling.
