Oz, you should take inspiration from this great guy called Bear Grylls. He was in the SAS and then he became the youngest Briton (aged 23) to climb Everest. The point is that in order to get sponsored to climb Everest he had to go around to loads of companies begging his case and phone literally hundreds of places, and most importantly he never gave up, not even when Richard Branson's housekeeper chased him and his friend away from Bransons house. He wrote a book about it, called "Facing Up", which you, and everybody else for that matter should read.
It's true that you need to first achieve something so that people will at least listen to you and getting your PPL at age 17 is something that will help. Then you have to build your flying experience and not give up when chasing sponsorship.
Good luck.