Where can you buy the shakes you are required to drink with the p90x?

I am buying the p90x workout program off of craigslist. It comes with the 13 dvds,nutrition guides,& the calendar. the one thing that im missing is, the shakes that you are required to drink in order to achieve p90x. What store can i buy the shakes at? And any substitutes to the p90x shake(different brands of shakes) would be great! im new to all of this,so i need details! thanks!