I found some stuff here -
The good stuff starts on the second page, under the heading 'RPG Map Symbols'. I haven't used any of it yet, but some of the medieval/fantasy buildings and stuff look like they'd be pretty cool in a small isomorphic-style map. I also found it was better NOT to download them - just copy the thumbnail into a folder (it's png, so you retain the shadows).
I'm thinking about extracting all the buildings and other terrain objects of my copy of HOMM4, but if I do that and make a map with them, I won't be able to post it (legally) anywhere, right?