Check with the Surrogate Court in the county where your mother resided to determine if the will has been submitted for probate.
If it has, you can get a copy. If not, you can probably petition the court to compel whoever has the will to produce it - I believe this is an option in NJ. Or file a petition to open probate yourself and ask the court to order whoever has the will to produce it.
(Until a person has been appointed by the court, that person is not the "executor." Someone may be nominated in the will to administer the estate, but until thee court grants the appointment, that someone has no authority.)