I found this website but forgot to bookmark it. You could browse the world by country and then by region and then by the type of map you were looking for. They had street, topographic, marine and I think sandbox maps. You could download each map in a couple of different formats including .img. I remember for California under topographic they had Monterey Bay, Yosemite, and Lake Tahoe. For other countries I remember Afghanistan, New Zealand and the list went on forever.