You will find out sooner or later: you must have your own bike when you plan to survive in the Netherlands. Some junkies started their own business selling stolen bikes at night time but buying one of theirs is considered handling in stolen goods and it’s illegal. The city has shops where you can buy second-hand bikes. Don't forget to get two proper locks: a wheel lock that is attached to the bike and a chain or cable to pull through the frame and front wheel to lock to a pole or tree.
You can find a very good bike shop in historical Delfshaven, Bergman Rijwielen, Spanjaardstraat 43 (second-hand bikes from € 100.-). The friendly owner even gives a 3 month guarantee on second-hand bikes.
Sometimes you can find secondhand bikes at the Blaak market or in the basement of Piekfijn thrift shop. The bikes you will find here are a lot cheaper (starting at € 60.-), but they haven’t been checked out.
You can also look online at www.marktplaats.nl, go to 'fietsen en brommers’, select 'fietsen dames' (ENG: bikes for ladies) or 'fietsen heren' (ENG: bikes for men). If you add your postcode you can look for bikes in your area.