I’ve had the best luck with BBBike and OpenMapChest for getting pre-built map files.
Basically you have to get one of these files with all the data you want in it and then stick it on your SD card on the GPS. (The GPS should mount like a thumb drive. If you already have a gmapsupp.img file on there, you might want to back it up in case things go sideways.) Some GPSes support multiple gmapsupp.img files, but a lot don’t. Here’s a thread on merging .
When I needed super fresh data, I’d download raw OSM data from Overpass and use mkgmap to build the gmapsupp.img.