http://www.nj.gov/state/dos_will_registry.html
This address will give you information about the NJ Will Registry and the form required to search for a Will. The Registry was created in 2005. Participation is voluntary and the program is not well advertised or known. Its too bad it doesn't work better. It is a long shot, but you might as well see if your mom's will is there.
If your mom is alive and is capable of making a new will that may be your best bet. Otherwise, if you have your mom's checkbook registers for past year scan through them looking for names of attorneys.
Good luck!
I agree with Paul. If that doesn't work, you may also be able to find the attorneys' name by searching her old check registries or bank-account statements. If that fails, she should get a new will prepared because a lost will might as well not exist.
Good luck!
An inquiry to the NJ Will Registry, and a thorough review of your mother's financial and other records are your best course of action. Alternatively, if your mother is competent, this may be a good opportunity to review her estate planning, and have her draw up a new will.