Stock up at the Whole Foods on the strip near the airport! I pick up my car at LAS, stop at the Whole Foods (which also has amazon package delivery), and then drive to my hotel. There's also a Total Wines and a few good restaurants in that mall (Town Square); that and Spring Mountain Road (Chinatown) are the best things about visiting Las Vegas.
The only nice thing I can say about Vegas is that I never had trouble walking up to a business from the sidewalk, asking for a cup of water and getting it for free. I think they didn't want me dropping dead from the heat right in front of them, that'd probably be bad for business.
Granted, that was 15 years ago and things may have changed. If I were made to go back, I'd bring one, maybe two, reusable water bottles and count on filling it myself in my hotel room.