Below is a decent list of major flea markets in Poland:
Here's another site with Polish biggest Antique Fairs:
One of the greatest flea market is in Gdansk during "Jarmark Dominikanski" (St. Dominic's Fair) - I'm very surprised that there's no reference about it in above articles. It's 3-week event and takes place every summer.
Gdansk Dominican Fair is the largest and most popular trading open-air event in Poland.
Long-standing tradition and a huge amount of attractions make it the largest event of its kind in Poland and one of the few largest in Europe. In 2013 the Fair was visited by 6.2 million people!
The main attractions of Dominican Fair is a trade offer over 1,000 stands with old artifacts, antiques, art products, handicrafts and regional cultural, artistic events and concerts.