“Eternal vigilance, as they say, is the price of freedom. Add intellectual integrity to the cost basis.”

Short story, 'Damaged Goods' - p.60
Short stories, The Turning (2004)

Source: The Genteel Tradition in American Philosophy (1911), p. 58

This has been attributed to Marshall, and he might have used the phrase, but earlier uses exist:
There is an imperialism that deserves all honor and respect — an imperialism of service in the discharge of great duties. But with too many it is the sense of domination and aggrandisement, the glorification of power. The price of peace is eternal vigilance.
Leonard H. Courtney as quoted in The Life Of Lord Courtney (I920) by G. P. Gooch
Courtney's statement however is probably derived from an earlier statement with several variants:
The price of freedom is eternal vigilance.
The price of liberty is eternal vigilance.
Eternal vigilance is the price of liberty.
These have often been attributed to Thomas Jefferson, but also Thomas Paine, Abraham Lincoln, and many others; Alfred Denning in The Road to Justice (1988) states that the phrase originated in a statement of Irish orator John Philpot Curran in 1790: "It is the common fate of the indolent to see their rights become a prey to the active. The condition upon which God hath given liberty to man is eternal vigilance."
Misattributed

[Chaturvedi, Pradeep, Women and Food Security: Role of Panchayats, http://books.google.com/books?id=IuKV5ak57asC&pg=PA46, 1 January 2002, Concept Publishing Company, 978-81-7022-873-8, 46–]

Speech in Boston, Massachusetts (28 January 1852), Speeches Before the Massachusetts Anti-Slavery Society (1853), p. 13. The memorable and oft-quoted phrase, "eternal vigilance is the price of liberty," was not in quotation marks in the printed edition of this speech. The Home Book of Quotations, ed. Burton Stevenson, 9th ed., p. 1106 (1964), notes that "It has been said that Mr. Phillips was quoting Thomas Jefferson, but in a letter dated 14 April, 1879, Mr. Phillips wrote: '"Eternal vigilance is the price of liberty" has been attributed to Jefferson, but no one has yet found it in his works or elsewhere.' It has also been attributed to Patrick Henry."
1850s
Context: Eternal vigilance is the price of liberty — power is ever stealing from the many to the few…. The hand entrusted with power becomes … the necessary enemy of the people. Only by continual oversight can the democrat in office be prevented from hardening into a despot: only by unintermitted Agitation can a people be kept sufficiently awake to principle not to let liberty be smothered in material prosperity.

Political questionnaire response (1952)
Context: I was taught that the American's right to be a free individual, not at the mercy of the state, was hard-won and that its price was eternal vigilance, that I too would have to be vigilant. I was made to feel that it would be a disgrace to me, as an individual, if I should not value or should give up rights that were dearly bought. I am grateful for that upbringing.

Farewell Address, (4 March 1837), recalling what, by then, had reached the status of a proverb.
1830s

How—and How Not—to Love Mankind http://www.city-journal.org/html/11_3_urbanities-how_and_how_no.html (Summer 2001).
City Journal (1998 - 2008)

On high culture and popular culture, in a letter http://www.scribd.com/doc/11510904/Adorno-Letters-to-Walter-Benjamin to Walter Benjamin (18 March 1936)