In order to download books from the US store you will have to change your country location to the US. You can do this from the "manage your kindle" page. You can do that while still in Canada. It's a kind of loop hole that may not stay open and may not always work but there are people who switch back and forth all of the time.
If you download a book from the US store, make sure you back it up on your computer in case the loop hole gets closed.
