I recently found a great source for maps from every state: the OpenStreetMap. You can download any portion of their maps in .osm format, and they provide tools specifically for converting them to Garming .imgs! More details on the process at their wiki.

They’ve imported and done some cleaning on the entire US Census TIGER dataset as well as integrating user-contributed tracks. I downloaded the .osm for my state, ran java -jar mkgmap.jar map.osm, and ended up with a .img better than the ones I used to piece together myself, and in a fraction of the time as well.

OSMs for every state in the union are here or here. Enjoy!

Update: I’ve converted each of the .osm files into Garmin .img format. You can get them (with the exception of California, Texas, and Virginia, which were too large to convert) at http://files.ideaharbor.org/maps/. Just download the .img of your choice, rename it to gmapsupp.img and place it in your Garmin directory on the GPSr, and you should have maps!