You can check local fuel station prices online before filling up, or take advantage of loyalty schemes. There are also ways you can make your car more fuel-efficient, so the fuel you do buy, lasts longer. For example:
keep your tyres inflated - your car will use more fuel if the tyre pressure is lower or higher than the recommended amount
lighten the load - if you can, avoid driving around with a full tank of fuel and a heavy boot
accelerate gradually - the harder you press on the accelerator, the more fuel you’ll use
change up a gear sooner - to avoid having to accelerate as much