How did the disciples of John the Baptist pray? Luke 11:1 (NIV) One day Jesus was praying in a certain place. When he
  finished, one of his disciples said to him, “Lord, teach us to pray,
  just as John taught his disciples.”


Since John the Baptist was the forerunner of Jesus who prepares the way for Jesus, there are many similarities between their ministries. John taught his disciples how to pray. Jesus also taught his disciples how to pray. But there is less information on the pattern of the prayer taught by John. If  Jesus taught "May your kingdom come", then John might have taught "Prepare us for your coming kingdom" or something like that.

Is there any source outside the Bible or some traditions?
This is an excellent question.  However, we are given nothing in the Bible that gives us any information about how John taught his disciples to pray, only that he did teach them to pray.  As far as I am aware, there are no surviving documents that give any information either.