I bought an accessoires kit for the eTrex on Amazon. I can't find the exact product anymore, but it was close to this:
https://www.amazon.com/-/de/dp/B07H89JBH5/r
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For maps there are tons of tutorials on websites and youtube, e.g.:
https://www.youtube.com/watch?v=8zA9zSQintk
As said, I used OSM. There is a detailed page what to do here:
https://wiki.openstreetmap.org/wiki/OSM_Map_On_Garmin
I used the so called "OpenStreetMaps Freizeitkarte" which is balanced between map size and details as well as POI etc. They are focused on Europe mainly, but other countries are included, too. And they are pretty easy to install: download and then there is an installer :)