You can purchase them on-line via the member website, by calling MS or at WDW.
If you purchase on-line or from MS, you will have the option to have an exchange voucher mailed to you or select will call. You will then have to go to a guest services location to get your actual AP. Your time begins when you exchange the voucher for the AP.
If you purchase at WDW, your time begins then.
Regardless of the purchase method, you will also need your membership card and photo ids for all adults showing the same address as the member.