Most of the travel within Turkey is done by using the bus system however more and more people are latching onto the idea of domestic flights within Turkey.
It is not often that it will work out cheaper however sometimes the cost can be as little as 20 dollars extra. If this enables you to get to your destination in half the time then it may be worth looking into to.
Here is the list of sites to use when searching for internal / domestic flights within Turkey.
http://www.sunexpress.com. This is my favorite site as the prices tend to be cheaper and they often have direct routes that the other airlines are not selling. You can choose your language from Dutch, English or Turkish and the search facility is really easy to use. I recommend to try the site before the others.
http://www.turkishairlines.com/en-INT/index.aspx Have you ever seen those annoying adverts on television with someone singing “we are Turkish airlines”? Well that advert is for this site. Even those their adverts are annoying, the site is quite useful. They display route maps and destination guides as well as having an easy to use search facility. Hit them at the right time, and you may catch one of their special promotions giving discounted prices on flights.
http://www.flypgs.com/en/default.aspx. This site is for Pegasus who operate international flights as well as domestic. The site is good for domestic flights only if you are using the major airports like Istanbul or Antalya. Constant promotions are displayed and sometimes it is possible to get a one way flight for as little as 40TL ( 27 dollars). I have used the airline myself and I was also very impressed with their level of service and customer care.
http://www.onurair.com.tr. Onur Air is the no frill cheap domestic airline for Turkey however I have found that on some occasions they have turned out to be more expensive than the other sites. They have the site in English as well but once again, unless you are traveling between major airports then you may find it a struggle to find a flight.
http://www.atlasjet.com – Readers recommendations . Please see comment below