You (currently) have two local services to choose from - BTL's Digicell and Smart. For Smart you'll have to buy a dedicated phone once you're here, but by repute the service is better and cheaper. For Digicell you may be able to use an existing phone so long as it operates on the correct frequency and is unlocked.
Whichever you choose will be a much better bet for use here than any service from out of country.