Not really. They do sell bottled water, $2.50 per 20 once bottle, no gallon jugs. Diapers are also sold, but usually only in small quanities, like 10 or so per pack, usually at $10 per pack. I've never seen baby wipes, but then I've never looked, again they'd be severely over priced, probably like $4 for a sample pack of 15 wipes. We always packed our own diapers and wipes, it always leaves room for those souviners you'll pick up during your stay. I always used the tap water for my kids and never had a problem with them, even in their bottles. If you don't want to carry all that stuff, you can either get a towncar to get you at the airport (most include a free 30 minute grocery stop) or rent a car and make the grocery stop yourself.
Edited to add: If you are a Pamper or Luv's person, you won't find any of those on WDW property. WDW only sells Huggies products exclusively.