The best way to get to Suan Pueng is by private car. Driving from Bangkok takes about two hours. You can take either Phetkasem Road (Highway 4) or Rama II Road out of Bangkok, both of which go all the way to Ratchaburi City. Then take Highway 3208 to get directly to Suang Phueng or Highway 3087 for a detour to visit a few caves before getting back on track at a junction right before you reach Suan Phueng Town.
As a border town, public transportation choices are rather limited. The farthest the train will go is Ratchaburi City, then you have to hop on a public bus to get to Suan Phueng.