I'm pretty sure they do not sell it anywhere. Possibly at the Hess gas station near Magic Kingdom toll plaza but that's it. If you are in desperate need, bring some with you before you go. Some rooms at WDW have mini-fridges.
I have never seen it anywhere in Disney actually, trust me I asked last time too...:lookaroun They have plenty of Amp though n I saw Monster down there about a year ago, never once Redbull though.
:shrug: Which hotels have it?