Where are the manuscripts preserved?

They are today subsumed within the Torah.

Moses wrote a scroll containing the narratives of Genesis, at God's command, before the formal Giving of the Torah (Rashi commentary, Exodus ch.24). It was this scroll which he read to the Israelites (Exodus 24:7) as an introduction to what God's covenant would entail. This was not a surprise to the Israelites, since they had carefully handed down the narratives and events of the lives of Abraham, Isaac, and Jacob, in small parchment scrolls (see Midrash, Shemot Rabbah 5:18 and 22).

After the Giving of the Torah, and Moses having written it in its complete form (Deuteronomy 31:24), the earlier fragments were no longer needed and eventually went lost.

See also:

Refuting the secular "multiple-authors" hypotheses