Where I can buy cheap used bicycle in Tokyo?

The cheapest way to get a used bicycle is to go buy one of the bicycles that have been impounded. 

This website shows when a and where you can buy one.  https://tokyocheapo.com/lifestyle/reconditioned-bicycles-the-two-wheeled-wonders-where-to-find-them/

Or you can always try the online auctions like Yahoo auction or Rakuten auction.