It's not necessary to get reals in advance if you're staying in the city, unless you're trying to get the first cab out of the airport. Just rely on your ATM card or use Amex/Visa. Visa is easily the most accepted. Note that a handful of places accept Visa Debit (Visa Electron) but not Visa credit cards. For example, buying tickets at pau de azucar or foz do iguazu. If you take a day trip someplace, you do need to take reals. For example I think there are no ATMs on Ihle Grande.