Here is a hint in the Bible: "Hail, full of grace!" said the archangel Gabriel. Who else in the bible is ever told they are "full of grace" by an angel? And how can she be full of grace if she is sinful. The translations refer to the Greek term kecharitomene, the past perfect participle of charis which means a gift, favor or grace. In Biblical Greek, this verbal form suggests permanence and singularity. Such singular, permanent grace in Mary is essentially the same concept affirmed in the dogma of the Immaculate Conception.
Another source of biblical evidence involves the references to Mary as "Woman" (e.g. Jn 2 and Jn 19). The evangelist alludes to Eve, who is called "Woman" in Gen 2. There are other parallels between the Genesis account of Creation and its Fall and the Johannine account of the Redemption. For example, the tree of knowledge caused Adam's death in paradise. The tree of the cross caused the death of Jesus, the new Adam, in Jn 19. So there is a certain biblical parallel between Mary, the Woman of the New Creation, and Eve, the Woman formed in original justice at the first Creation (i.e. before the Fall). This parallel is stated explicitly by very early Church Fathers like Justin Martyr (d ca 160) and Irenaeus (d. ca 220). None of this is explicit proof of the doctrine. However, it is solid support from Scripture alone.
Then there is this from a 16th century theologian who wrote a little prayer book in 1522 in which he wrote:
"She is full of grace [voll Gnaden]; so that she may be recognized as without any sin. ... God’s grace fills her with all gifts and frees her from all evil."
In a 1527 sermon this theologian proclaimed:
"It is a sweet and pious belief that the infusion of Mary’s soul was effected without original sin; so that in the very infusion of her soul she was also purified from original sin and adorned with God’s gifts, receiving a pure soul infused by God; thus from the first moment she began to live she was free from all sin”
The name of this 16th century theologian: Martin Luther. And he wrote this and preached it years after breaking with the Catholic Church. Although Lutheranism does not today declare the immaculate conception to be doctrine, they do call it a "pious belief". (not sure what that means - perhaps you could clarify, Bonnie?)