Can i buy from amazon.com(US)?

Yes it works, you can register normally but just use US address and that's it, you're safe as long as you don't go yelling all over the internet that you're doing what you are. I've bought a lot of deals over AmazonUS.

Some keys are region restricted to the US. Dead Island, Riptide, and Binary Domain. There may be others.

If its region restricted to the US.. .Will it work in Canada still?

I bought the dvd versions of some games in US or UK and activated them in germany.
worked great.
you only have to be sure the keys are not restricted to some countries.

generally you're ok but random games may be regionlocked to north america without being labeled because it doesn't matter "us only" so check first

Use Borderlinx, that's all. And except some region restricted keys, all other steam keys should work fine.

Use HotspotShield when you activate games which are resctricted in your area. It works in Steam, dunno about Origin and others.