There is a number of public Busses stopping close to the Suan Pakkad Palace Museum. If you are not familiar with the Bangkok Bus system it is the easiiest to just take a taxi. It's in the middle of town so it will not burn a hole in your wallet. ALternatively you can also take the BTS Skytrain and get off at station "Phaya Thai". From there it is just a 5 Minute walk down the road.