While I have little trouble coming up with adventure ideas for my home campaign, I'm not that pleased with my map-making skills.  So I often crib maps in whole or in part from other sources or even purchase maps or map collections from online sources.  I got some great maps back in the 90s from Wyrm Works and I've picked up others over the years. 

Among the best maps in modules I've re-purposed are those from Iron Crown's MERP game.  I'm actually thinking about using their massive Dol Guldur module and re-purposing its maps for Mount Vhuurmathidon since its description in the rulebook of warrens with strange monsters leading to cyclopean deeps seems to fit how Iron Crown designed Dol Guldor.  I think I'll use Orthanc for the Tower of Ghul.  I also use real world stuff.  I'm trying to find a schematic of Angkor Wat for a yuan-ti/snake man city in the jungles of a fallen empire in my regular campaign world. 

Just curious if others had suggestions for good map sources.