There are a few hotels that will change money for you even if you're not a guest of theirs, but failing that there is a bar/luncheonette in Copacabana that will change your money and will actually give you a better rate than most official change places.
The bar is called O Crack Dos Galetos and is located on one of the smaller roads running in off the Copacabana beach, near the Meridian Hotel. There's an outdoor restaurant on Avenida Atlantica near the Meridian Hotel called Mabb's. The street next to Mabb's is where you'll find O Crack Dos Galetos.
Walk into the bar and go to the cash register. The guy at the register will change all your money for you and will tell you the exchange rate first.

