It seems the “border crossing card”: http://travel.state.gov/visa/temp/types/types_1266.htmlis only for Mexican Citizens.
It really seems like as a Canadian you can go back and forth with no trouble, so I am not sure you need anything. There are special types of things for Canadians doing business in America. I would call the border, or US immigration and try to get some information. If you have a lot of troule getting to a live person, don’t press any buttons like you are on a rotary phone. They will likely answer in Spanish, just tell them you speak English, they are all bilingual.