The website allows you to not only book tickets, but choose your seat. It also gives the date and time of day when booking opens, so the true opera fanatic goes on to the website at precisely that moment to get the best choice of seats.
OK, any luck?
Try their website
But if you are in Stockholm, you may well visit their box-office
Sorry, i can't give a website per tripadvisor rules. But check your search engine for Royal Swedish Opera - Kungliga Operan, and you should be able to find the correct link with English translation.
There is a lot of booking information on this page.
Get quick answers from The Royal Swedish Opera staff and past visitors.