Where can one find box and send something home in Vienna? We will be staying near Museum Quarter.
You're looking for a post office (die Post). You can buy all sorts of postal supplies at the post office. The post office I know of is that at Westbahnhof. One entrance is directly opposite from a Chinese restaurant on a side street perpendicular to Mariahilferstrasse. There is also another post office on the very top floor of Westbahnhof ie, inside the Westbahnhof passage of shops opposite of Müller, a very nice inexpensive shop that sells tons of things.
I think that from the box you can send only letter or postcard. If you want to send a package you have to go to the post office.
As noted above, you want a post office. They sell boxes and everything you need to send a package.
There are locations all over town, and there is one very close to the MuseumsQuartier. It is located at Mariahilferstrasse 22-24. It is open M-F from 8 am to 6 pm, Saturdays from 9-12, closed on Sundays. You want to be sure to send your package certified or registered mail. The one at Westbahnhof mentioned above would be out of your way.