I found a video showing where the microSD card slot is - on a Nuvi 3590 device (which may be what you have). If you have a different model, the slot may be in a slightly different location.
NOTE: You can see the slot at about 45 seconds into the video.
Click Here --> https://www.youtube.com/watch?v=6kg7okIwRXY
That should be the slot you want to use for an extra microSD card. And by the way, these units can handle UP TO about a 32GB card, but I would think an 8GB or 16GB card would be "plenty" for what you need.
I did a search on Amazon for an 8GB microSD card, and have the results below.
8GB microSD card SEARCH
Please let me know if that is the slot you are looking at or if you have any more questions.
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