Q. For MLA citations, how do I cite a source that is mentioned in another source?

Whenever you can, take material from the original source, not a secondhand one.  Sometimes, however, only an indirect source is available -- for example, someone's published account of another's spoken remarks.  If what you quote or paraphrase is itself a quotation, put the abbreviation qtd. in ("quoted in") before the indirect source you cite in your parenthetical reference.  If you do not take direct quotes, just cite the source you have.

Example:

Samuel Johnson admitted that Edmund Burke was an "extraordinary man" (qtd. in Boswell 450)

Then on the works cited page:

Boswell, James. The Life of Johnson. Ed. George Birkbeck Hill and L.F. Powell. 6 vols. Oxford: Clarendon, 1934-50.