To find free return address labels, you can write to a charity, such as March of Dimes, the Veteran's Association or the Wildlife Association, and receive address labels in return. Often, if you subscribe to magazines through a service, the magazine publisher mails out personalized address labels as well.
Charities often send out letters to potential donors that contain address labels with a design pertinent to the charity's function. Many times, the charity continues to send address labels long after you cease contact with the organization. Beware of magazine subscription services, however, as they often sell your information to third-party advertisers.