I don't have any special online resources to recommend but I do suggest that you try to snap up out of date atlases; they are often sold off by libraries and schools, and occasionally end up in second hand bookstores. Also, maps of large parks and local area maps are useful; I recommend writing to rural chambers of commerce and asking them to mail you maps of local parks, recreation areas, etc. Usually they'll mail you a big package for free, typically with 3 or 4 maps useable for RPGs. Regional offices of government departments of Forests, Fisheries, Parks or Tourism also often have a wealth of free, useful maps.
The best set of RPG maps I've obtained for free so far was sent to me by the chamber of commerce for Mackenzie, BC, Canada.