Tirta Gangga is a popular side trip from the nearby coastal resort towns of Amed and Candidasa. Organised tours are widely offered.
Public buses run from nearby Karangasem town and Perama (http://peramatour.com) buses can be chartered from Candidasa.
If you are driving yourself, Tirta Gangga is on the main east coast road just north of the town of Karangasem Amlapura and is fairly well signposted.