The market is located by the river, near Memorial Bridge. I went early in the morning, when it was still relatively cool. The streets and warehouses in the area are filled with fresh flowers (some sections also have veggies) at really cheap prices. I bought five sizeable bunches of different coloured orchids for a grand total of THB 120!
I also got really lucky with my timing and caught the early-morning aerobics class in the flower warehouse. Catchy tunes, an energetic instructor and an enthusiastic group of flower sellers dancing in sync - it was quite the sight! A few minutes in, everyone in the warehouse was shaking a leg, even the old ladies manning their stalls.
Alternatively, if you're going late at night or early in the morning, take a taxi. Tell the driver "Pak Khlong Talat".
Opening hours: 24 hours, 7 days a week
Tip: Smile! It'll get you better prices and all the photo permissions you desire. Case in points: the lovely lady posing with the birds of paradise below.