You can get a BB from either T-mobile or AT&T...either will work for Claro. These phones use a SIM chip, and are the easiest to get connected with Claro. You can get them anywhere, not just at the stores.
If you think your wife will use internet on her BB then I suggest the one from AT&T, because Claro and AT&T share the same frequency for the 3G internet so it will work much faster. T-mobile phones work on Edge with Claro and internet will be much slower.
If it already comes unlocked that is great, but don't pay anything over $15-$25 to get it unlocked. There are tons of places to unlock in SD and I think they charge approx $5-10 USD.
As an aside, have your wife be careful how she walks around the streets of SD using that phone. They are very popular, and a woman flashing a nice shiny new BB might be an opportunity that some thieves might not pass up.