The Bloemenmarkt floats on barges along the Singel between Koningsplein and Muntplein, and has been selling flowers here since 1862. The stalls carry cut flowers, potted plants, herbs, and — most usefully for visitors — packaged bulbs you can take home through customs. Tulips, hyacinths, and dahlias are the main draw. The corner where Singel meets Koningsplein has the freshest cut flower stalls.
Buy bulbs in season (July through Christmas) and choose loose bulbs over pre-packaged ones where possible — you can inspect them properly. Even if you're not buying, it's a pleasant stretch to walk through on the way between the Rijksmuseum area and the city centre.