If you buy Pampers* diapers, Amazon is the way to go. Through their Subscribe and Save program you get 30% off. You sign up for a recurring order (I do monthly) and a week before it ships they send you an email reminding you of your order so you can change or cancel as you like. You can also have the order come early, but it takes a few days to get here after you order it.
*I am not in any way endorsing Pampers–as a Pediatrician I have no opinion as to which diapers are best. As a mom, Pampers fit my little girl well and if I can find a good deal on them…even better.