The Ramkhamhaeng National Museum (พิพิธภัณฑสถานแห่งชาติ รามคำแหง) is in Sukhothai, and naturally its principal focus in the ancient Sukhothai kingdom. Khmer items make up a very small part of the collection, but three of the ruins in the ancient city were originally built by the Khmer and later adopted and modified by Sukhothai kings, so there are some.
As of February 2019, these are all the Khmer artifacts from a known temple ruin on display.
Mueang Si Thep (เมืองศรีเทพ) – Amphoe Si Thep, Phetchabun
Wat Si Khao (วัดสีเขา) – Amphoe Mueang, Nakhon Sawan
Wat Si Sawai (วัดศรีสวาย) – Amphoe Mueang, Sukhothai
Details – 9am-4pm, 150/30B foreigners/Thais, 055-697367
Location – Mueang Kao, Amphoe Mueang, Sukhothai Province