Begin your search online with Spanish low-cost airlines Spanair, Vueling and Air Europa. All three connect major cities, including Barcelona, Madrid, Seville, Valencia and Ibiza.
Consult other discount companies like the United Kingdom's Ryanair and EasyJet, two of the top low-cost airlines in Europe. Both operate frequent flights from a number of locations in Spain.
Don't write off the country's largest airline simply because it's not low-cost. Iberia offers domestic flights for less than 30 Euros (about $40) each way plus a convenient traveling experience as it operates from major airports and presents customers with more amenities.
Compare the deals you've found on the airlines' sites with the offers at booking companies such as Rumbo and LastMinute, which compile options and prices, including those for train travel.
Verify that you're not missing out on any additional discounts by calling the airline directly.