Richard Dawkins and Christopher Hitchens discovered as their book tours brought them through the very buckle of the Bible Belt that wherever you go, there are plenty of people who are happy to be and know atheists. But you're right, finding them can be a pain if they don't come to you.
Honestly, it's worth bringing up your atheism early if you're worried it will end a relationship. Sure, it might last longer if you don't, but you'll always be worried.
My first port of call would be the internet. www.richarddawkins.net has a social section, there's regularly an ad on the right of this site for www.freethinkermatch.com, and many of the conventional dating sites have an option to search by religion. (Yes, I know atheism isn't a religion, but having it on the list is better than nothing.) There are lots of other communities out there.
Offline, just go out and meet lots of people in any setting which isn't explicitly religious. Don't expect everybody to be devout, because trust me, they aren't.
Finally, don't think you can only find happiness with another atheist. My girlfriend is a liberal Christian, and although we disagree on many things we're happy in an atmosphere of tolerance, personal respect (as opposed to respecting all each other's views) and free exchange of ideas.
- SmartLX