it's at the us border office you pass before you enter canada. at the border we go through, the office is across the street from the duty free shop. there is usually a sign that says "export authority" or something like that. it's best to call the office you plan to export at and ask them the best way to get to them.
If you contact the specific office, they will send you an information pack that details exactly what to do and how/where to do it, as it may differ between border crossings.
Here's a thread that will give you all the info you need. It is quite recent.
http://www.canadavisa.com/canada-immigration-discussion-board/-t415713.0.html
