As far as I know the majority of tickets havent gone on sale yet, just what they call 'earlybird' tickets which are made available to people who bought tickets last time. You can register on the T website so you will be alerted as to when tickets are going on sale.
i got mine at double8.com they are based in london i think they have a shop there if you pay for them now you get them cheaper the nearer it is to t in the park the price shoots up i was sent my tickets three weeks before the event last year good luck.