Where can I buy the Paris Visite ticket? (I live in Brazil). Also, how do I calculate whether it is going to be cheaper than just buying single tickets?
My opinion is that a Paris Visite pass is hardly worth the money.
If that's true in your case is difficult to answer. Basically, you have to use the public transport quite heavily. The price of single metro ticket is 1.70 EUR and if you buy tickets by a batch of 10 ("carnet"), the price goes down to 1.33 EUR per ticket. With that information in mind, take the Paris Visite pass you are interested in (1, 2,3 or 5 days), and calculate how often you would be able to take the public transport on regular tickets, ot how many discounts you would have to use.
Example. A 2 day pass is 15.85 EUR. For a bit less than that, 15 EUR, you can buy 11 regular tickets (1 carnet and 1 single ticket). That's a lot for two days, especially if you plan your sightseeing a bit rationally.
And don't let yourself fool by the discounts. Some are more sexy, some other are less sexy. Check precisely what the discounts are good for, check the prices. And check if you are interested in these at all! Plan your sightseeing first, i.e. make a list of what your are interested in and see if that's covered by the pass. Don't do it the other way round. You might miss a lot!
From outside of Paris:
You can book it online directly at Rail Europe.
Paris Visite is a tourist trap.
Consider getting a Mobilis. This is what the locals use when they need to travel for an entire day. It's like the Paris Visite, without the airports' main rail access (Orlyval and RER B at CDG), there is no voucher booklet and it is much cheaper than the Paris Visite. It is available only in 1-day version. You can buy one for each day of travel, and modulate the cost by buying only the zones you need for that day.
Paris visite is only available in zones 1-3 or 1-5 version. Mobilis allows finer control.
The obstacle to get it is that it is not sold at the airport stations... maybe you can use a single-trip ticket to quit the airport then use Moblis tickets during your sightseeing, or look for airport shops that sell it.
If you stay from Monday to Sunday, buy a Navigo Découverte (smart card) and load it with a weekly pass.