The Library of Congress has an online catalog of over 6000 photographs and graphics from the decade of 1950 to 1960. The library catalog listing provides known copyright information about each item, including contact information.
The historical photographic site Shorpy.com also has an excellent collection of 1950s material. Some images are selected from the Library of Congress archive, but many are unique to the site. Shorpy curators archive user submissions of historical interest and develop photographs from old negatives, plates and slides. Using the photographs for commercial purposes requires permission, and non-commercial use should cite the source.
Stock image libraries, such as Shutterstock, provide 1950s photographs. The images have watermarks with the source name, a standard practice among stock image libraries. Removing the watermark requires a paid account.