Most of them these days are culled from the cream of the existing crop in the lower divisions.
Most of the bigger cup teams will look at hundreds of drivers each year, planning for a new driver in 5 years. Then slowly invest a little money and time in that driver as he (hopefully) moves up therough the ranks and eventually makes it into a a truck or the busch series.
Your odds are better of getting hit by lightning, but if that'* what you wanna do, go for it. I would.