Pork Belly in northern NJ
If you go to most Asian Markets there are butchers that specialize in pork cuts. Some markets naturally are better than others. The Chinese Market in River Edge on Kinderkamack Road has a good one and is a privately owned store. The larger Korean owned Corporate Supermarkets will have pork bellies as well, but slightly higher in pricing per pound. Little Ferry and Englewood are probably the two best stores.
As an added bonus if you are looking for fish, they all have excellent fish departments as well.
There is a market in Hackensack, Giant Farm, but the quality of meats is not as good and the fish prices are higher than the others mentioned.