Alicante's Airport is located just 43 km. long from Benidorm in one of the most enterprising and dynamic areas of Spain. It offers you the chance of enjoying modern and equipped installations and services. Most of the low cost companies fly to Alicante. There you can go by coach to Benidorm in around 40 minutes.
Link on this site and find which company is the best for you
Here is a link to book a Shuttle bus to go to/from the airport to/from Benidorm at very reasonable price
Valencia's airport is good option too. It's around 90 minutes driving
Here is a link for Valencia's low-cost flights
If you need more info about Benidorm, go to the link  below