I don’t know about buying it, but there is a link on my Orthodoxy page with the whole book online. There’s also a link on that page to the Jordanville monastery itself, which may help if you would like to buy the hardcover book itself.
If one wishes to buy a copy of the Jordanville Prayer Book (I own two now--one I inherited from my mother of blessed memory, who used it daily), you may usually find one or more in the parish bookstore in your friendly neighborhood ROCOR church. Parish bookstores in some other jurisdictions sometimes carry them in stock as well.
Hypo-Ortho