According to their tweets, in-store pre-orders start as soon as the store opens.
Online pre-orders start around 12 PM EST. I suggest spamming the F5 earlier.
------------------------- My WTB thread
Buying Zelda CIB NES/SNES/N64 games in mint conditions! (Both NTSC and PAL)
Also looking for Phantom Hourglass NFR Demo (PAL) and Twilight Princess NFR Demo (PAL)
I've ordered a few things from the American GameStop site in the past. You pay around $20 for shipping, but at least you can get it.
And then, you can always cancel your pre-order and order on the Canadian EBGames site if it becomes available there later on. That's what I did with my Shulk amiibo.