Well, I suppose the only way, really, is to find a family that has kept on the tradition since those times. My girlfriend said someone from her church (Wicca) came from a family in Europe who hid from the Pagan persecutions by pretending to be Catholic, all the while keeping the tradition. The sad part is, they tend to stay underground because of the social stigmas against pagans. I'm sure you'd really have to peruse some crazy-old libraries and look for days, weeks, years, just to find something valid. Kinda like in the movies
But unlike in the movies, it's likely you may not find anything, as they always passed on their teachings verbally.