MLA Citations (9th ed.)

The publisher is the organization primarily responsible for producing the source or making it available to the public. (p. 164)

If two or more organizations are named in the source and they seem equally responsible for the work, cite each of them, separating the names with a forward slash (/). But if one of the organizations had primary responsibility for the work, cite it alone. (p. 170)

Omit business words when you give publishers' names in the list of works cited, including Company (Co.), Corporation (Corp.), Incorporated (Inc.), Limited (Ltd.), etc.

In the names of academic presses, replace University Press with UP (or, if the words are separated by other words or appear alone, replace them with U and P: "U of Chicago P").

In all other cases, write publishers' names in full. (p. 172)

A publisher's name may be omitted in the following kinds of publications:

• A periodical (journal, magazine, newspaper)
• A work published by its author or editor
• A web site whose title is essentially the same as the name of its publisher
• A web site not involved in producing the works it makes available (i.e. YouTube, WordPress.com, JSTOR, etc.). These sites may be named as containers if the contexts are organized into a whole, but the site still does not qualify as a publisher of the source.

See p. 165 in the MLA Handbook Ninth ed. for more information.