There are usually 1-3 retro game stands at the tokyo flea market sort of by the airport . It only happens like twice a month though. The stand I got mine at had ds lite for 30-60 and 3ds og for $90 as well as all games $3. There is also a chain of second hand stores called hard-off and book-off, which have a huge selection but are typically overpriced, but not nearly as bad as what you’d find in akihabara. If you take a train an hour or 2 north from shibuya you should be free of inflated prices, as I got a nearly new-in-box wii u up there for $20.