http://bt.spaceempires.net/index.php
Most of the stuff isn't available as torrents, but the SE torrent index is here:
http://bt.spaceempires.net/index.php
The other good place to look is on PBW (
http://seiv.pbw.cc/)
All the mods that people play multiplayer are available up there (since the PBW server processes the turns for you, they need to be
)