Digicel stores are only found in Jamaica and a few other Caribbean countries. However, once you have a Digicel phone/sim card you can roam in north America (in the US you can roam on the AT&T network. not sure about Canada).

I once found a third-party website that would sell you an activated digicel SIM card, loaded up with minutes and ship it to your home. However, the cost was twice as much as going to the digicel store in JA. I was also afraid to order something by mail and find that it didn't work in my phone. It was much cheaper and convenient to stop at the digicel store in the Burger King plaza and get my old Blackberry setup by someone who knows what they are doing.