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.com galleries and archives.
The Art & Map Gallery and the Map-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 Archive, which has all the maps from the 4e Dungeon and Dragon Magazines.
