Go through your results for flights to San Sebastian in Spain with Netflights. Although Vueling usually has the best prices, it's best to double check what's available for your travel dates.
Flexible on dates? Conducting your search by month allows you to compare prices to San Sebastian throughout the year. Accommodation prices will also be higher at this time of year.
So, if the flights to San Sebastian don't work for you, you could always try searching for flights to other airports in Spain.
Looking at a few different departure airports will give you even more choice. See what you can find. And remember, it's best to book your flights in advance wherever possible.