Sources behind Mark's Gospel
Mark would have received information from secondary sources (he wasn't an eyewitness to events). The sources would have been written and oral but Mark probably relied more on oral sources because books in Roman times were scarce and expensive. It is likely that many of the sources were secondary and didn't witness events themselves (although don't forget that he knew Peter who would have witnessed events).
Mark's Written Sources
1) Some of the stories about Jesus and his teachings would have been written down so that preachers could use them. The construction of Mark's Gospel suggests that he used these sources e.g. In chapters 2 and 3 there is a collection of stories relating to Jesus's disagreements with the religious authorities. In Chapter 4, several parables are grouped together.
2) The Passion Narrative: Many scholars think that the Passion narrative (Jesus's crucifixion) would have been written down early on in the Church's existence because it was so important.
3) Ur Markus: Many scholars think that Mark worked from an earlier…