I don't see any nintendo eshop cards in Canada, which can be delivered online.
If I buy such card in US, will it work in Canada?
I understand from this discussion you can't use US eshop card if your console set to Canadian region. Although you could do the following:
You have two options. You could format your 3ds and set the region to US(you will lose all data in your SD, system memory, and will unlink your NNID from your system excluding preinstalled games if you bought a limited edition 3ds) or you could ditch your US eshop card and buy the Canadian one.
Anyway I found where I can buy Canadian eshop card, so problem is solved.