Sao Paulo-Rio is the second-most traveled flight pattern in the world (Madrid-Barcelona is number one). Don’t worry about reservations - flights leave at least once an hour, and are usually more like once every half-hour. There’s even a dedicated ticket desk for “buy & fly” purchases -which shouldn’t cost you more than $100 US.
Now, don’t get on the next plane - at least not if you can’t get a window (that’s the whole point). Wait until the one after- you should pretty much have your pick of seats. Although both sides actually have great views, my advice is to sit on the left-hand side window. Rio’s domestic airport (Santos Dumont) is a little two-runway job right next to downtown. This isn’t the difference between flying into JFK and LaGuardia - flying into Santos Dumont is basically the equivalent of flying into the Wall Street Heliport.
Not only are the views astounding (and way better than my disposable-camera picture above), but you can grab your stuff and walk right into the middle of Rio. The subway - which will take you as far south as Copacabana - is only about a 15 minute walk through downtown.