They also regularly do cycle tours round the area. They're very friendly and helpful too, much better than the TAT is in my opinion. The building is right by the steps of the Pin Klao bridge (Saphan Phra Pin Klao).
Visit the Bangkok Tourist Bureau at 17/1 Phra Athit Road (not too far from National Museum and alongside the river). They have several brochures for bikeways and trails in Bangkok and beyond.