Prague's only real fleamarket 'Bleší trh' happens every Saturday morning in Kolbenova (beside Kolbenova metro).
A huge sprawling outdoor market, you'll find broken electronics, piles of clothes, paintings, communist items, furniture, jugs, quirky souvenirs, jewellery, and the odd amazing antique find for a Czech price.
It's 20 czk you pay in at the turnstile (under a euro) and well worth the finds.
Get there early (7am-2pm) and bring a phrase book so you can haggle with Czech prices. Don't expect English.
There's also plenty of sausage stalls and trailer-bars when you want to stop for a beer (pivo).
Bleší trh fleamarket in Prague
Kolbenova, Praha 9
Kolbenova (on metro line B)
Google map: tinyurl.com/yb8gl25