Ok, yeah, don't stay in a crack den or on the streets
There will probably be somewhere at the airport. I dunno about prepaid... from what I understand, it's different here than in the states. You'll get the SIM for free and have to put money on it, or buy it with whatever you pay already put on usually. If you're calling home a lot it's good to go with something that offers good call rates on out-of-country things. Otherwise, with just the sim card, most providers are pretty much equal.