Can a buy a prepaid SIM card while I'm still in the US?

I'm going to Bahia this fall for a year and I was wondering, that if I unlocked my GSM could I get a prepaid SIM card while I'm still in the US? That way, all I would have to do is switch it when I get off the plane instead of wandering around in confusion until someone picks me up.

If not, will the airports most likely have shops selling phones? (No, I'm not sure what airport I'm coming in through yet.)

Someone is right up there, quadriband mobiles work fine.
In Bahia international airport you can get SIM cards, but go for the carrier "OI" it's the best GSM in THAT area. As well as Rio and most of the northeast. Since OI owns the landline concession in the whole Northeast, Rio and Minas Gerais, they have better prices for cell phone calls to and from.
Costs about 10USD and there are several places you can buy, including the Airport. But I think you need a CPF, which is sort of the Social Security in Brazil.
So it should be easier to get someone to buy it for you, like a cab driver. Due to cell phone smuggling into prisons in Brazil there is tighther control now.
you can get a CPF whitouth a ID card and you cant get an ID card without a resident/work visa.
Don't use TIM, it sucks.

North America, Canada and a few other countries in the Americas use a different set of GSM frequency band than the rest of the world. Brazil mobile networks operate on 900MHz and 1800MHz frequency bands and are hence incompatible with American mobile handsets.

I recommend these folks; you can either rent a phone or buy one (for about $50, a deal even if you don't travel a lot). Then you can just buy a Brazilian SIM card, which will give you a local number.
http://www.mobalrental.com/

They also sell SIM cards, but the phone charges per call might be steeper than buying a card when you arrive.

TIM (the best provider) kiosks are in all major malls and airports and department stores. Note that after you buy a TIM SIM chip, you will have to call and register your account (as part of its anti-crime laws Brazil does not allow anonymous cellphone accounts). You will need to give your name and passport number. Cards that allow you to add credit to your account are available at newsstands throughout Brazil.

yes! search TIM brazil sim cards on google.

i use TIM and i travel to brazil every month or so from the USA.

I even got a cheap phone here for 50 bucks, but ive used my unlocked GSM phone from the states.

However, i think it might be a little cheaper to buy the sim card here - as I did, but I could be wrong.

The sim card here was i think 19 reals if I remember.

Most GSM phones sold in the U.S. are 麓Quad-band`and are compatible with frequencies used in Brazil. Before leaving for Bahia I contacted my carrier (AT@T) and they emailed me instructions on how to unlock my phone. In Brazil I was able to puchase a prepaid SIM for less than U.S. $5.00.
Shops and kiosks for the various cellular companies are located in most airports ( and everywhere else.)

Ruby.
In this popular brazilian website link, you can to buy a simcard from Bahia.
This store, send a international delivery and sale credits prepaid.
This simcard already have R$10,00 in credits.