i found additional maps for alien swarm

http://www.filefront.com/17154634/AS-Little_Map_Pack5.zip/

1.Just click the link or copy paist
2.then click the yellow download bar
3. itll save as a zip, unzip it then restart steam
4.then go to the folder and click on each of the maps and it should say successfuly installed then ok and continue to the next one
------if it says failed to install try the next one. my computer was a b**ch so i had to restart before they would install----
5.after you install all of them(i had about 4that failed) restart steam again
6. then start alien swarm and go to options and see if they are under add-ons.
good luck hope it works..