There is a Digicel retailer at the MBJ airport. It's in the outbuilding on the left as you exit the arrivals hall, where the taxis and buses congregate. I had to wait a few minutes. It's a very small store and there's only room for a couple people at a time inside. You could also ask your driver to swing by a Digicel store on your way to Negril.
Know that your phone has to be "unlocked" for the new SIM card to work. Typically, to unlock your phone, it has to be paid off in full (not making monthly payments for it on your bill). Then, your service provide can give you a code that needs to be entered into your phone to unlock it.
Johio
Thanks so much!
Picked one up at the Digicel store near Burger King. Have had no issues!