Dear friend,
Only God is the origin of grace. God gave Mary grace at her very conception because of the channel she was to be from Him to the human race as the mother of His Son. It was in her very womb that the Word was made flesh and dwelt among us. When we say that she is full of grace, we mean that she has never been with sin—even original sin.
So, grace does not originate in her. But her prayers reflect her holiness as she intercedes with her Son for us. She has been doing this ever since that bride and groom found themselves without enough wine at their wedding (Jn. 2:1-11).
Fr. Vincent Serpa, O.P