Walgreens has a store in Las Vegas NV and Walgreens sells Adult Diapers and they have their own brand of Walgreens Certainty Briefs.
Other than that there's also changing times that sells adult diapers or you could even ship some of your diapers to the hotel where you're staying if you're staying at a hotel or take enough Adult Diapers with you on the trip to Vegas to last you.