Siam Market (full name - Siam Night Market and Garden Market), is on Sukhumvit diagonally opposite Bangkok Hospital Pattaya. The plants and flowers are the sole feature all day Friday-Sunday. It gets very busy at weekends, so Fridays are best if you can make it. You can get practically anything there, from potted plants to towering trees. Great for a wander, regardless of budget, with food and drinks stalls to keep you going.

Orchids 60 baht each or 2 for 100.
Smaller ones ​are 25 baht each.