Austrian MotoGp Tickets

Hello, I'm kinda new around here and I need the help of someone who's more experienced than me!

The next month I will be traveling through Austria from Italy with my Monster and I was planning to stop on sunday 14 at the redbull ring to see the race. The problem is that on the official site there's no way to buy the tickets and I don't know which site to trust to buy them, or if I should just buy them once there...

Can someone help me? Thanks a lot!