Opera

If the music doesn't work out, you can still do a tour of the building most afternoons.

Here is the official website to buy tickets for the big opera house.
https://www.wiener-staatsoper.at/en/season-tickets/events/
Closed Sept 18
Guenther Groissboeck recital Sept 19
La Traviata Sept 20

La Traviata is most likely sold out already, but you may try to get onto the waiting list. In the very expensive hotels the concierge may have connections to obtain tickets. Do not buy tickets from a scalper on the street.

You may watch the performance outside the opera building on a video wall for free:
https://www.wiener-staatsoper.at/en/your-visit/service/opera-live-outdoors/

La Traviata is indeed sold out. However, if lighter fare (and lighter on the wallet) is OK, the Volksoper is doing "Die Csardasfuerstin" on the 18th. It is an Emmerich Kalman operetta from the early 20th century. In German, and I don't know if they do English supertitles at the Volksoper.

Groissboeck is a bass singer with just piano accompanist. But prices are good if you just want to see the inside of the opera house during a performance.

In German, and I don't know if they do English supertitles at the Volksoper.

Yes, they have for most performances (see schedule).
https://www.volksoper.at/schedule/september-2018.en.html

Any suggestions how and which site to get tickets for a reasonable
price.?

In Euros, what specifically do you consider a reasonable price?

Then go to the links wmt1 provided above and upthread to find your price range & then buy your tickets. We went to the Wiener Staatsoper (Vienna State Opera) and the experience was sublime! Have a wonderful time wherever you go.

In Vienna there is nothing like the TKTS booths in Manhattan or Brooklyn where you can buy discounted tickets.

The tickets come in different price categories, you choose what you can afford. Always buy the tickets from the venue's website or at the venue itself. The federal funded venues (e.g. Staatsoper, Volksoper, Burgtheater, Volkstheater, etc.) have a central ticket office next to the Opera ("Bundestheaterkassen") and a common website for booking: https://www.culturall.com/?language=2

Buying tickets from privately owned ticket agents will imply a hefty surcharge. Never buy tickets from scalpers on the street.

The cheapest way to watch a performance in the Opera is getting a standing-room ticket 90 minutes before the opera is commencing.
Read the respective description of the procedures: https://www.wiener-staatsoper.at/en/your-visit/ticket-sales/ticket-sales