I mean, Nevada has very permissive gun laws: no purchase permit, gun registration, or owner license required, open carry permitted, "shall-issue" for concealed carry permits, no magazine or rifle restrictions, no waiting periods, etc.Needs a manifesto for something like this.
How do you get an arsenal in a casino? No metal detectors? It's a casino, with money and drunk people.
So guns and ammunition are very easy to get, and he probably just carried duffel bags full of them right up to his room.