OpenStreetMap can be used to create free maps for your Garmin. You could download those maps from services like OpenStreetMap.nl. Those maps contain a lot of unnecessary features (houses, woods) that make it hard to follow your route (see the screenshots). So if you want a map that makes it easy for you to keep on track, follow this guide.
You could use XAPI Query Builder to draw a bounding box and download your maps, but that frequently fails because servers are overloaded. It's better to download a recent map from Geofabrik.