There are places such as the following in CJ that offer what you're looking for: http://store.babycenter.com ( get 6% back ) http://www.babyearth.com ( get 8% back ) Those two above are under the Babies section under Family category. There are probably others but I stopped looking after I found those above.
I have to ask, do you actually know that there is a market of people buying diapers and baby food online? granted I don't know anything about it, aside from having younger siblings, but I can't think of a situation where someone would buy those things online. walmart=cheap diapers unless maybe it was some super healthy earth-people type of baby food.