The cheaper Staatsoper tickets go on sale one month before the performance date - so if you wanted to go on say, 15 October, the cheaper tickets will go on sale at www.staatsoper.at at 9am Austrian time on 15 September (sometimes a few minutes before then!) The two most expensive categories go on sale on 1 June for the whole year. That is why at present only the expensive tickets seem to be available.
The agencies like Viennaticket.com buy up blocks of tickets on spec, I believe, and that is why they offer the cheaper ones in advance of the Staatsoper site. But if you buy from them you will probably pay 20% markup on the price. I prefer to wait until the ticket sales go live and buy them direct from the venue.