Is this commercial or just for fun?
There are lots of Google sketchup models of the Temple of Jupiter, Pantheon, etc. Rhino can import export. But you can't use many of these commercially.

Here's a good shape library. Lot's of high rez like a bust of Nero, statue of Neptune.
http://shapes.aim-at-shape.net/viewmodels.php?page=1

Free meshes and more low rez buildings here. The index has a text search.
http://www-c.inria.fr/gamma/download/

Roddluc2001 has some midevil market and half timber buildings.
http://www.rodluc.com/download.htm