If you have lost a bird see the links below. Also immediately call veterinarians, rescues, humane societies, animal controls, pet stores and etc to notify them of your lost bird. You can often post flyers at these venues as well which may help put you in contact with people who have found birds. You may want to consider doing this across several counties as cockatiels (and other birds) can get quite far with unclipped wings, fear, and a good gust of wind.
For both lost and found birds, place ads in newspapers. If you have found a bird, do not post too many details. For example say, "found cockatiel, call to identify" or "found: bird, call to identify". There are many people who will call up and say they lost a bird just to try and get a free one (so if you post: "Found: cockatiel male with yellow face and orange cheeks with white on his wings" and a person calls up and says that is their bird...how will you really know?").
Places to post lost or found birds:
Tips and Information on finding a lost bird:
Winged Wisdom Pet Bird Magazine
Good source of cockatiel related information
HolisticBird.org
This is an interesting site if you are interested in natural bird care
What Should I Feed My Pet Cockatiel? (The National Cockatiel Society)
Feeding Cockatiels (The National Cockatiel Society)
Choosing an Avian Veterinarian (American Cockatiel Society)
Tips to Keep Your Cockatiel Happy and Safe (American Cockatiel Society)
Wing and Toenail Trimming (American Cockatiel Society)
Recipe Corner (American Cockatiel Society)
Cockatiel Health (American Cockatiel Society)
Large number of cockatiel articles on health and various diseases