As Canadian can I purchase cell phone and pre-paid cellular service in USA for six week period?

Cross by car through Montana, Idaho, Utah and Nevada on our sense for stay of 4 weeks to Phoenix in the winter. Don ‘t know much about the phone system in the United States. We want to avoid costly roaming charge them so won ‘t be using our Canadian cell phone. [Telus] Verizon or any other company has program pre-paid national can get evit diare roaming charges while travelling through these conditions and during our stay in the Phoenix area?
As Canadian I can buy pre-paid attend Helena Montana for period of six weeks? Also as Canada, can buy the phone and assisto in line before our departure?
Thank you