Amazon has become one of the best places to buy Diapers - especially if you can get your hands on one of their 20% off codes! Combine that with the Amazon mom discount (30% off subscribe and save - you can cancel after it ships) and you have a GREAT deal you don't even have to leave your house for. Luvs diapers usually yield the BEST deal!
Here are a couple of FREE baby Magazines that will often have the codes in them
Parent magazine also often has the, and there are always deals to get a subscription for cheap (I listed one a couple weeks ago)