City Navigator is copy protected and must be purchased for each device. Any discussion of ways to circumvent copy protection is not allowed on this site.
Garmin sells the map as a download, but you must connect the desired GPS during the download process and it will then be permanently locked to only that device. So, if your GPS breaks or is lost then you're just out of luck. They also sell City Navigator pre-installed on memory cards. These cards can be used on any GPS, however, only one at a time. The cards are copy protected and cannot be duplicated.
I had a detailed map of NJ available here in the past, but I no longer make maps for Garmin devices. However, all my maps can be used on your phone with my free mobile web app. Here's one example that covers everything South of I-195, but there are many other maps available in the app.
https://boydsmaps.com/#17.00/39.881838/-74.534055/pines2020/0.00/0.00A big update is coming for my web app in a few weeks, with lots of new features and full compatibility for desktop computers as well as phones.
For your GPS, you could look at the NJGIN trail map which was one of the data sources for my map. It can be downloaded as a KML file which I *think* Basecamp can import and export back out as a GPX file for your Garmin handheld (I haven't ever tried however). If not, then Google Earth should be able to open it and export as a GPX file.
https://njogis-newjersey.opendata.arcgis.com/datasets/f866ab117bb64288a7de0b07c206cc70/explore?location=39.885006%2C-74.499243%2C12.23Of course, that would just appear as a "track" on the GPS, and you would need another map to use as background.