There are many things “hidden” in Sacred Tradition from which arose the Scriptures we have today–as confirmed by various Church councils. One of the things the Church had meditated upon down through the centuries by her theologians, scholars and saints is Mary’s place in the salvific economy, IOW, what place she had/has in God’s plan of salvation.
Mary’s person, her role, her meaning to Christians was one of those things left to the Church to delve into as one of the treasures of the Gospels. Jesus didn’t settle all questions while he was here on earth teaching his disciples. If you read Acts 15 you’ll see that a very important matter had to be decided right off the bat by the early Church in its first council.
Also, you should understand that the Bible is one of three witnesses to the teachings of the Church. The other two are Sacred Tradition (oral teachings given through preaching, etc. and the Magisterium or teaching authority of the Church. So, Catholics don’t go to the Bible hunting for verses to “prove” this teaching or that. And neither did the writers of the Gospels. Whenever they cited a fulfilled prophecy they simply stated that it was a fulfillment not that it “proved” anything–do you see what I mean?
When converts come from Evangelical/Bible based faith traditions they don’t see the Bible from a Catholic/historical point of view. They think of the Bible as an “authority”, but no book, no matter how sacred is an “authority” because authority lies in persons not in objects. IOW, the Bible cannot tell us: “Yes, you have correctly interpreted this passage or you have the right idea from that verse”–only persons can do that, which is why Jesus set up his Church to decide matters of faith and morals instead of dictating a book to his followers.
Anyway, there are OT passages that bear witness to Mary’s Immaculate Conception, as interpreted by her holy men and women and confirmed by the declarations of the Church. Some of the most beautiful ones of this sort are found in Song of Songs, such as:
As a lily among the thorns, so is my love among the daughters. (Sngs 2:2)
Thou art all fair, O my love, and there is not a spot in thee. (Sngs. 4:7)
One is my dove, my perfect one is but one. (Sngs. 6:9)
My sister, my spouse, is a garden enclosed, a garden enclosed, a fountain sealed up. (Sngs. 4:12)
There are other passages, some from Proverbs and others from Sirach (one of the Deuterocanical books).
It might help if you were to make a study of Marian doctrine and dogmas. But, the most important thing you have to do is to reorient your thinking to the way the Church views her own book, the Bible and her own history. That will be a big help not only in understanding Marian teachings but all the others, as well. God bless you! I will pray for you as you go through RCIA.