You can fly and/or drive to Page Lake Powell. There are no buses, such as Greyhound, that go between Flagstaff or Phoenix and Page Lake Powell.
The Page Municipal Airport has an Avis Car Rental, and Flagstaff and Phoenix airports have a plethora of car rental options. If you fly into Page, you can take a taxi to your destination or, depending on where you're staying, there may be a shuttle service available (ask your hotel). Flights between Flagstaff and Page are about an hour and $100. Flights between Phoenix and Page cost about $150.