Forewarning: Forgive me if I'm missing dashes, accent marks, capitalization, anything of that nature.
The more I think about this, the more it confuses me. Where do the Zanpakuto actually come from? At first glance, it seems like they show up when a person gains their Soul Reaper powers... except Tosen took his from a dead comrade, Kenpachi had his long before he became a Captain, and we're shown lower-level and in training Soul Reapers with what I believe were even explicitly called "Training Zanpakutos."
So... I'm confused. Unless the answer is that they're magic swords that you either have or you don't.