You won't have a problem finding fruits and veggies.
On the way from Mobay to Negril you'll find at least one large market area in each town you pass through.
Around Negril...there are quite a few trucks selling fresh produce...just stop and check to see what they offer. There's also a regular market up the road behind Yellow Computers near the round about.
I find during the slow season (actually during high season also) the freshest produce can be found at Quality Traders aka the Green Store.