@Sviatogor Yes oral tradition is important. Romantism gave a strong boost to this branch of science. “Ethnic spring” during 19. century was very important in understaning own culture and traditions. Sources from that time are important eventhough they are mostly bad interpretaded. That was the beginning of ethnography so people often made mistakes. But nevertheless very important part of research.
Saxo Gramatico and other christian early medieval writers who lived near slavic tribes can give us also very important resources about our old religion. For me the work of Toporov and Ivanov (Russians) is the base of modern science about slavic mythology.