There are literally hundreds of maps for use with d20pro. Here are some of my favorite sites to snag from.
Dundjinni User Forums: Maps
http://www.dundjinni.com/forums/forum_topics.asp?FID=6
This site really cannot be complimented enough. With over 1800 posts on this section alone, you will find HUNDREDS of maps for your campaign. Browse or use the search function!
The Mad Mapper
http://www.educatedgamer.net/gallery/
My own hosted site for maps from me, Supercaptain, and Cisticola. My maps are ok, but Supercaptain and Cisticola are simply incredible. This site has a full section of maps for some Dungeon Crawl Classics, plus some Dungeon Magazine maps.
WOTC Map Gallery
http://www.wizards.com/default.asp?x=dnd/arch/ag
Not finding enough maps? Grab a few hundred more here, from the official D&D producers. Note that they are organized by sourcebook, so you can easily find the maps for Red Hand of Doom, or Castle Ravenloft, for example.
RPG Mapshare
http://rpgmapshare.com/
Lots more maps made by enthusiasts. Some here are superb, with very high resolutions.
Creative Gremlins
http://www.creativegremlins.com
A premier map site that just started. Content is growing, and is done by some of the best mappers out there. Click on the Gallery and the Marketplace.
