I just came across an excellent article on how to download free maps to your Garmin Edge 705/800/810
OpenStreeMaps are easy to order from this site http://garmin.openstreetmap.nl/
and DCRainmaker tells you how to get them onto your map-capable Garmin device here:
http://www.dcrainmaker.com/2013/05/download-garmin-705800810.html