Free flea market as in don't need to pay rent for the stall space?
As far as I know, maybe it is only Sungei Rd Thieves Market. Most of the flea markets you will prob have to pay a small fee.
There's FleaFlyFoFun @ Home Club, MAAD Mkt @ Red Dot Museum or organisers like Flock to shop, For Flea Sake, Bazaars Inc etc that you can contact to set up shop. They organise flea markets arnd S'pore.
Heartland Flea at Pasir Ris Sports and Recreation on 24 March 2012 from 2-8pm by Fleanatic event. More info on facebook or http://thefleanatic.webs.com/