how are they sourced?
The few known facts come from archaeological digs, foreign travellers' accounts of the Rus' land, and linguistic comparative analyses of Slavic languages
East Slavs
Researchers know relatively little about the Eastern Slavs prior to approximately 859 AD when the first events recorded in the "Primary Chronicle" occurred. The Eastern Slavs of these early times apparently lacked a written language. The few known facts come from archaeological digs, foreign travellers' accounts of the Rus' land, and linguistic comparative analyses of Slavic languages. 

Very few native Rus' documents dating before the 11th century (none before the 10th century) have survived. The earliest major manuscript with information on Rus' history, the "Primary Chronicle", dates from the late 11th and early 12th centuries. It lists twelve Slavic tribal unions which, by the 10th century, had settled in the later territory of the Kievan Rus between the Western Bug, the Dniepr and the Black Sea: the "Polans", "Drevlyans", "Dregovichs", "Radimichs", "Vyatichs", "Krivichs", "Slovens", "Dulebes" (later known as Volhynians and Buzhans), "White Croats", "Severians", "Ulichs", and "Tive
who would call the sea of japan the east sea South Korea where is this sea? It is bordered by Japan, Korea (North and South) and Russia. where did the last mentioned place take its name from? The name "Russia" is derived from Rus', a medieval state populated mostly by the East Slavs. who are there people you mentioned at the end? The East Slavs are Slavic peoples speaking the East Slavic languages.
wiki:11560152
East Slavs [SEP] Sources
Researchers know relatively little about the Eastern Slavs prior to approximately 859 AD when the first events recorded in the "Primary Chronicle" occurred. The Eastern Slavs of these early times apparently lacked a written language. The few known facts come from archaeological digs, foreign travellers' accounts of the Rus' land, and linguistic comparative analyses of Slavic languages. Very few native Rus' documents dating before the 11th century (none before the 10th century) have survived. The earliest major manuscript with information on Rus' history, the "Primary Chronicle", dates from the late 11th and early 12th centuries. It lists twelve Slavic tribal unions which, by the 10th century, had settled in the later territory of the Kievan Rus between the Western Bug, the Dniepr and the Black Sea: the "Polans", "Drevlyans", "Dregovichs", "Radimichs", "Vyatichs", "Krivichs", "Slovens", "Dulebes" (later known as Volhynians and Buzhans), "White Croats", "Severians", "Ulichs", and "Tivertsi".