They both are. Sparks has no web site, I called (via a recommendation from a friend). Totally legit, even got pulled over once and my papers were in fine order. Call Omar for a quote: 876-312-8351.
Island Cruiser: http://www.islandcruiserjamaica.com/
Those plus Vernons are the only ones I am aware of in Negril. Island Car (not Cruiser, the big company), and probably some of the other ones with multiple locations, will deliver a car to Negril but that costs about the same as you'll pay for your transfer.
