Every resolution that you can choose in your windows desktop is also availible in the Source engine without the need of any modification. So what you need to do is to add the custom res you want to use in Source to your windows desktop. Check this out if you have a nvidia card:
http://www.widescreengamingforum.com/wiki/index.php?title=Nvidia:_Add_Custom_Desktop_Resolution
If you have an ATi card you can try Powerstrip. I don't know if you can add custom res with an ATi driver. Maybe you can do it with ATi tray tools but that is only a guess. Don't shoot me if i'm wrong :D.
Take a look in this thread to find different res: [url]http://www.widescreengamingforum.com/node/#comment-]