I read that a nazir would shave his hair just prior to taking the vow because of a decree of the sages. Is it true? What is the source for this information?
There is no trace of such a decree, not in the Mishna, nor in the Rambam.
I don't remember the parsha or pasuk. But it isn't a rabbinic decree it's a biblical obligation. Refer to the parsha that talks about nezirot for a full answer