As of April 2015, the website CaliforniaGasPrices.com allows users to track gas prices across the state. The data displayed on this site is aggregated from hundreds of users across the state and accurately reflects current prices.
To find the cheapest gas prices, use the sort function on CaliforniaGasPrices.com to rank the listings from low to high. Remember to choose the correct grade of gasoline. Furthermore, it's important to sort the results by zip code. You'll need to consider the trade off between how much money you save and how far you have to drive to reach the gas station to determine if the cheaper price is really worth it.
The United States Department of Energy is also a good source for finding information on Californian gas prices. The Department of Energy hosts a page on its website that lists all the cities in California and provides helpful links pertaining to their gas prices. This resource is quite intuitive to navigate.
Another source for California gas prices is Motor Trend. The site features zip code specific information on current gas prices. Motor Trend updates its prices on a consistent basis for California's major cities, but sometimes fails to collect information on gas prices in rural California.