http://www.mediamax.com/theromero/Hosted/quake/quake_map_source.zip
http://www.quaddicted.com/files/quake_map_source.zip
http://www.inside3d.com/idmapsources/quake_map_source.zip
The files weights about 2.3MB.

From the readme:
These are the original Quake map source files as of Quake v1.06. I've included the chopped-off beginning to E2M6 (The Dismal Oubliette) as well, named E2M10.MAP. The majority of these maps were created during the last 7 months of Quake's development cycle when we had decided on the final direction of the game. For a comprehensive listing of each map, its name, author and place in the final game please visit the Wikipedia (http://en.wikipedia.org/wiki/Quake).

Also included with the full map sources are the map sources created for the game's items (armor, ammo, etc.) which were needed to light the BSP files so the items could be viewed in the game.