Accompong's link doesn't work.
DO NOT CHANGE MONEY AT THE AIRPORT! The rate is horrible.
DO NOT CHANGE MONEY ON THE STREET! You are just asking to be short changed.
Use a reputable cambio. We have used FX Trader for years. They have locations island wide. There's one on the strip in MoBay and one by the HiLo in Negril. There are others as well but FX is our favorite.
You can also use ATMs. Some give $US, some give $J, some give either.
$US is accepted everywhere but it is usually easier to pay in $J.
My rule of thumb is if the price is listed in $US pay with $US, if it's listed in $J pay with $J.
There is a link to a handy conversion chart on the home page of negril.com.
Small bills of either currency are always helpful for tips and taxi's.
Enjoy!