There are two open air flea markets in Zagreb.

One is a fancy event that happens every Sunday morning at Britanski trg (alongside Zagreb main street Ilica) where smart looking people show off their knowledge and interest for antiquities. Good craftsmen exibit reconstructed items and if you are short in time it is worth the effort to see it.The square is a nice place to have coffee & watch the market live and breathe.

Second one is Hrelić on Sajmišna cesta, alongside Sava river (in new the part of the town - Novi Zagreb) and it's an exibition of the Croatian reality. It happens twice a week - Wednsday and Sunday mornings. Everybody sells everything (from jewelry & clothes to used books, vinyls, toys, bikes, furniture, etc) and unbelievable bargains and findings are possible. The mess on Sundays is increased by the used car market in the neighbourhood. It is worth your effort to find the direct bus at the bus terminal at the main railway station and go there. Another possibilities are a taxi or a 15 minute walk from the tram stop. Beware - visitors start showing up as early as 7 or 8 AM (if you are on treasure hunt you should be there as early as possible) and it closes around noon.