Rather than attempting to find tourists going to Ochi on your own, you might check with the drivers who do tours whether they have a party going to Dunns River Falls during your stay, and see if you can share the ride.

Or just schedule with the drivers doing airport transfers for a ride Negril-MBJ then MBJ-Ochi, and back the same way.

This becomes much simpler if you're not trying to arrange a party of 5.