In Rio the 3 main cel providers are Tim, Oi and Claro. I got an Oi Sim (no real reason for choosing them) a couple of years back. I had to replace it last week and got a new number. Cost was R$20 which came with R$20 credits. I purchased it in Lojas Americanas in Rio Sul mall in Botofogo as the Oi store and kiosk there were out of prepaid (pre-pago) chips. So far so good.

However unlike last time when I registered in-store using my passport the registration process is now on the phone and requires a CPF (Brazilian national identity no.). I used a friend's but this may be a problem if you're a tourist and/or don't speak portuguese. So in short ask in-store about the registration process before purchasing.

Also the latest incarnation of the Oi Sim seemed to be eating up the units. The most cost effective way to use the Sim is to receive calls and only make essential calls on it. Otherwise purchase a phone card (cartao de Telemar) in a newspaper kiosk and use it for outgoing calls particularly if calling a landline 021-2xxx-xxxx rather than 3xxx-xxxx, 8xxx-xxxx, 9xxx-xxxx etc.

All 3 providers should have stores near where you're staying whether it's either in Copa, Ipanema or Leblon. Otherwise try Lojas Americanas although the dedicated stores are more likely to have English speakers.