Wizards.com itself has a massive collection.
Finding exactly what you need can be tedious, but for sheer volume it's hard to beat the wizards.comwizards.com galleries and archives.
The Art & Map GalleryArt & Map Gallery and the Map-A-Week ArchiveMap-A-Week Archive are both free, and give access to most of the maps published in D&D 3.5 physical volumes and adventures (though not the content that goes with them) as well as some bonus maps. If you have a D&D Insider subscription you can also access the Article ArchiveArticle Archive, which has all the maps from the 4e Dungeon and Dragon Magazineshas all the maps from the 4e Dungeon and Dragon Magazines.