There are many gas stations in town and one at the resort which is very expensive. Shop in town for best prices. Also, if you are driving up from Phoenix you will be on the Navajo Reservation from Flagstaff to Page. Cameron or The Gap have the best gas prices. Gas is always less on the reservation where taxes are lower. Gas up on the reservation to save.
Page has a Walmart superstore on Highway 89 and a Safeway grocery store in Page.