Flight Tracker in WPF, C#

Hello everybody,.
 
I am implementing a flight tracker project in WPF and c#,
I would like track some flights in according to latitude and longitude in real time.
There are many application that do this, but i dont know where they take the data.
 
I have two question:
- Someone can you tell me where i can find the database, or data source, that i can query to extract in real time the data?
 
- These database are free?
 
Thanks so much, any help is usefull.