7-11, gas stations, truck stop and these types are better suited for energy drinks. Personally, I have never, ever seen anyone buy an energy drink from Walmart. And although I'e never been in Costco's, I would bet that you don't see many people buying energy drinks there either.
So although Walmart would be good, I'd much rather see chain's like "76 Truck stop," "Sheetz" "BP" and other large chain small convenience stores and gas stations.
In fact, forget Walmart, put this in all "BP's" and we would be off and soaring.