Here you can download a bike map of the United Kingdom which was created using data from OpenStreetMap. OpenStreetMap works that way, that everybody can upload their tracks. This creates maps from all over the world map just from track data.
That means that the quality of the map depends on everybody (you're welcome to contribute to the project by uploading your tracks as well). From that data you can basically download a Garmin map from every place in the world.
Check here or here for maps from other places or for the .img tiles of all over the world to include manually in your GPS. Here you can also find a list which refers to websites which offer ready to use maps from OSM data for your Garmin device, and gives instructions on how to choose the right files and how to include them into your GPS device.
As there are all the time new tracks being contributed to the projects the maps are usually updated weekly to monthly, so you might want to check every couple of months for newer versions of the map.
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