Tickets can be tough, as most are sold on a subscription basis.
If you haven't already, check the web sites of the Philharmonkker and of the Musikverein. Tickets are not sold by the Philharmoniker itself, but rather by the organizer (the Musikverein company itself, in this case). On the latter's website, it states that tickets for non-members go on sale three weeks before the event date.
You could also, of course, let your hotel's concierge obtain the tickets for you (a surcharge above face value will apply).
There are two concerts on the 8th, both conducted by Mehta. The roster of singers for the Mozart Requiem in the evening concrt looks good.
http://www.wienerphilharmoniker.at
http://www.musikverein.at