While I will agree with Dorinda that our local Costco/BJs/Sams club is a great place to find affordable diapers, I also love buying diapers from Diapers.com.
Their cost per diaper is very competitive. They also have a HUGE selection of brands and sizes. The best part is that they ship directly to your front door. If you spend $45 or more, shipping is free!
I usually receive my order within 24-48 hours of placing my order. I have never paid shipping.
They do accept manufacturer's coupons. Once you mail them in, they take about a week to credit your account.
I've also had great success using their referral system. If you use my "code":
you will get $5 off your first order, and I will get a $1 credit on my account. I average a $5 credit/month based on my referrals. Every bit helps!