Near Phra Chan Ferry Pier, by Thammasat University.
National Museum (Phiphitaphan Sathan Haeng Chat) Na Phra That Rd, (next to Thammasat University, facing Sanam Luang).
Taxi is probably the best way to go. Or embark the Chao Phraya Express Boat to Maharaj Pier, then walk about 20 minutes.