University library would be your best chance. I don’t have experience with history papers, mostly scientific and medial and sometimes I have to pay a subscription to an online publisher if I want more than just the abstract.
I found the program for the conference:
http://www.acmrs.org/conferences/MAA_2011/MAA11ProgramFinal.pdf
Here’s the professor’s email address and details: http://nd.edu/~medinst/faculty/bios/boultond.html
If you can’t get it from the University, the paper may be picked up and published in a number of journals, like his three previous papers so you could write to them and see if they’ll be publishing it…
Recent Publications
Alta Studia Heraldica: The Scholarly Journal of the Royal Heraldry Society of Canada, vol. 2 (2009), and vol. 3 (2010).
The Ideology of Burgundy: The Promotion of National Consciousness 1364–1565 (edited with Jan Veenstra, Brill, 2006)
The Knights in the Crown: The Monarchical Orders of Knighthood in Late Medieval Europe, 1326–1520, second edition, revised and expanded (Boydell and Brewer, 2000)