Many hotels have a few hanging around, left by other tourists. Check where you are staying before you buy one. Unfortunately, a few of the hotels actually try to charge a rental fee for use of floaties left behind by their guests...I would pass on that...better off buying your own and offering it to local kid on the beach when you leave. They will be thrilled to get one!
It is probably going to be a hit or miss buying inflatables in Negril. For years I have been bringing rafts and assorted water toys. My wife always says to me how much of this stuff are you going to bring? Masks. snorkels, fins, spears, rafts, what next! Next trip im eyeing a personal water scooter to pull me around the reef. It weighs only 12 lbs. We'll see. IMO it would be easier to find what you want to use in Negril and bring it with you. More of a selection and definitely much cheaper here in the States. Internet has so many sites to buy stuff. Try leisurepro.com and scuba.com. Most sites tell you what the product weighs so you can judge the weight of your suitcase. Sometimes i leave stuff with the locals which they appreciate instead of taking it home. Enjoy and have fun. Dan