Oxforddictionaries Online:
expedience
noun
Fowler states that both are valid; expediency more common.
expedience, expediency.
Both forms are in use in the meaning 'fitness, suitability, advantage', although expediency is much more common (three times more, according to the evidence of the OEC). The rhythm of the sentence often determines which is used:
The present mentality on the island emphasises short cuts, expedience and disdain for professional standard --New Scientist, 1991 /
At the time there was a case to be made for the new policy satisfying the demands both of expediency and humanity --K. Tidrick, 1992.
I like Fowler's observation on rhythm of the sentence.
Anne Bronte, The Tenant of Wildfell Hall 45-1
"It is no question of mere expedience with me; it is a question of life and death."
[EDIT]
Jacky Wong
Dalian Wanda: An Offer That Makes Shareholders Want to Move On The Walls Street Journal, May 30, 2016
Plan to delist Wanda’s commercial-property unit so soon after IPO reeks of expedience rather than long-term value creation (emphasis added)
Leonard Pitts Jr., When expedience feels like wisdom, The Seattle Times
Froma Harrop, Confessions of a Former Dead-Ender, May 31, 2016. RealClearPolitics
It all seemed so unfair. Hillary the workhorse had labored at putting together a coherent health reform plan. The glamorous Obama floated by. Political expedience prompted him to oppose an individual mandate -- unpopular because it forced everyone to obtain coverage but absolutely essential for universal health care. (emphasis added)
Mike Selvey, Alex Hales shines as England survive Sri Lanka’s catching masterclass, The Guardian, 27 May 2016
There was a missed opportunity for James Vince too, victim, for the second time, of his own enthusiasm for stroke play over expedience. (emphasis added)
Expedience is not even dead: nGrams