Mike, to purchase, simply peruse the dealer listings on the main page. But it sounds like you are unsure as to which Sub model you want. That being the case, you will save yourself alot of frustration &, perhaps, buyer remorse, by researching the model you actually want before hitting any buy buttons. I would recommend either spending some time on timezone.com or 1 of the various Rolex forums to narrow down your target Sub. Or, better still, visit a local Rolex dealer & spend some wrist time getting accustomed to the various models. You may find that 1 stands above all others. Or, you may even find yourself fast-forwarding to purchasing the real thing (as many here end up doing).
Good luck & happy hunting.