Just because a source is unpublished or unofficial, it doesn't mean you can skip citing it. Even if you are simply quoting an unpublished manuscript, you still have to give credit. In the MLA style, some unpublished sources, like emails or a thesis, have specific citation guidelines, but if there are no specific guidelines available, you still need to include as much information as possible to show where the idea or phrasing came from. To do otherwise would be considered plagiarism.
Citing Generic Unpublished Work
According to the seventh edition of the MLA Handbook for Writers of Research Papers, if your source is unpublished and doesn't have a specific citation system for that type of work, then you would use the following format to properly cite your source: Author (last name, a comma and then the first name). Document Title (italicized). Date of Composition (or n.d. for "no date"). MS or TS (indicating if it is a handwritten "manuscript" or typed "typescript"). Location of Material (Institution, Location or Collection Name or "Author's private collection").
For example: Doe, Jane. Pizza Toppings (italicized). N.d. TS. The Pizza Museum Collection.
Unpublished Work with Specific Citation Rules
Some unpublished sources have specific citation rules. For example, a personal e-mail should be cited like this:
Smith, John. "Re: Nabokov's Butterflies." Message to author. 17 May 2012. E-mail.
An unpublished thesis or dissertation also has its own citation method:
Smith, John. "Butterflies in Literature." Diss. Duke University, 2013. Print.
Always check in the style guide to see if there is a specific way to cite your source before using the generic formula for citing an unpublished source.