There are several places to go to find phone numbers, including dialing 411 on your phone, but the most used are directory assistance sites such as 411.com, AnyWho & ATT's Directory Assistance page. You must know the name of the person or business and the general location.
In addition to finding phone numbers through directory assistance resources, it is also possible to find phone numbers for various businesses on their own web pages. Some larger chains include search functions to locate stores in your area and provide contact information. Some companies may also provide contact information for executives and/or departments within the organization.
When looking for phone numbers for individual people, however, the resources may be less effective and often cost money. In addition to directory assistance options, you may also use resources such as PeopleFinder.com. These resources search various public records to obtain information about individual people including addresses and phone numbers. These sites generally produce long lists of similar names, and it can be difficult to identify the records pertaining to specific individuals. These sites generally charge a fee to access the information their sites locate, but this may also be true of the primary directory assistance sites, especially when dealing with unlisted phone numbers.