Hawaii does not have all the private, low-cost gas stations you may be familiar with on the mainland.
Traditionally, (pre COVID) Costco gasoline on the Big Island had the cheapest price. Since COVID their price was closer to the cheapest non-warehouse price. The website HawaiiGasPrices.Com shows current prices (Use Zip 96740 in your search). You must be a COSTCO member to buy gasoline or shop there.
As our island is rural and quite large, it is suggested that you ensure that you have sufficient fuel for your travels. You could find yourself driving 45 minutes to the next station.