Well, I would say first that your question presupposes that Scripture is the only authentic source of revelation in Christianity, and that would be wrong from the Catholic point of view. The issue has been discussed at *great* length :-O on FR by myslf and many others before, so I won't belabor it much now. Suffice to say that we Catholics, who gave birth, as it were, to the New Testament canon directly through both Tradition and the magisterium of the Church (the "other two" sources of revelation), consider the concept of Mary being conceived without sin and preserved in grace to remain sinless throughout life to be a "settled matter" going back to the early Church.
Anyway, there IS supporting Scripture for the concept. Mary was greeted by the angel Gabriel as being "full of grace," in the present tense. Since this event clearly took place before Calvary, and, especially at the time, it would be impossible for any human being to be addressed in this way without a singular grace being accorded to the person by God, it follows that something singular was involved in Mary's circumstances.
The last verse of Revelation 11 talks about the ark of God's covenant appearing in God's temple in heaven; the first six verses of chapter 12 clearly represent Mary. Remember, there were no chapter and verse divisions in the original text; they are often rather arbitrary. Here's a perfect example. The ark and the "woman clothed with the sun" are one and the same. The ark of the covenant was the holiest object in Israel. Its elaborate embellishment was meant to attempt to make it worthy of the emblems of God, wrought by His hand in the case of the tablets and the manna. Both the ark and its contents were prefigurements to Mary and the One that she was a living ark to for nine months. How much more worthy would she be to house the Word of God Himself, instead of things that were mere things that prefigured Him?
Again, I understand that you likely aren't too impressed with an argument based on Tradition as authority. But the early Church was. You have to ask yourself by what authority was the principle of Sola Scriptura (itself not specified in Scripture)enacted at the expense of other sources of authority known by the Church from the beginning.
If you would like a full treatment of the subject from a Catholic perspective, I would suggest the following website: http://www.newadvent.org/cathen/07674d.htm
"I would say first that your question presupposes that Scripture is the only authentic source of revelation in Christianity, and that would be wrong from the Catholic point of view....
I understand that you likely aren't too impressed with an argument based on Tradition as authority. But the early Church was. You have to ask yourself by what authority was the principle of Sola Scriptura (itself not specified in Scripture)enacted at the expense of other sources of authority known by the Church from the beginning."
If we both accept the premise that Scripture is "God's Word," how then can the "tradition" of man be as reliable a source of truth?
Man (and papal edicts) and "tradition" have changed in the past and still do, whereas the the Word never changes.
"We Catholics... consider the concept of Mary being conceived without sin and preserved in grace to remain sinless throughout life to be a 'settled matter' going back to the early Church."
Having been raised a Catholic, I understand the doctrine.
While the term "full of grace" or preservation of "sinlessness" throughout her life is not provable, nor scripturally specific, Mary was indeed "blessed amongst women."