For instance: from Jesus’ mother, whom they knew.
John knew her particularly closely as he took Mary into his home and treated her as his own mother, and provided for her after Jesus’ death, as Jesus had asked from the cross.
The apostles knew Jesus’ Mother during the years Jesus was with them…and if in the company of Jesus, would you not, also, have chats with the mother of this remarkable man about His early life? Wouldn’t you have wanted to know everything she could tell you?
And a couple of the apostles hung out with John the Baptist before Jesus called them. Remember that Mary spent time with Elizabeth, her cousin, for a few months before John’s birth. Another family connection and information source. Elizabeth would have shared, as mothers do, with John when he was younger.
God bless you