You would do best at ATMs in Brazil. (However those at GIG are known to be compromised.) You can either change enough for a taxi at the airport exchange booth (not good rate) or take a pre-paid by credit card taxi, until you can find an ATM. There are also 'cambio' offices in Copacabana if you bring cash (dollars/euros preferred). Credit cards are widely accepted for hotels, restaurants, and even small purchases, but keep the card in sight at all times when paying, and carry it in a place secure from pickpockets.
Have a good trip.