I've read several articles saying that Diet Coke is sweened with
Stevia in Japan and Canada. I am looking for a place where I could buy
Stevia-sweetend Coke online and have it shipped to the US.

If these sources are accurate, Stevia is no longer used in Diet Coke in Japan:

"It is also used in beverages, including (until recently) the Japanese
version of Diet Coke."

http://www.myvitanet.com/strebonow.html

"Japan originally made their diet Coke with Stevia, until they decided
to standardize with the rest of the world."

http://www.mnwelldir.org/docs/immune/immune1.htm

"Stevia is widely used all over the world. in Japan, for example, it
claims 41% of the sweetener market, including sugar, and was used in
Japanese Diet Coke until the company replaced it with aspartame to
'standardize' worldwide."

http://www.healthfree.com/Dr.JulianWhitakers.htm

I could not find Stevia Coke anywhere, but I recently found this Sevia
Black Cherry soda...

http://www.sodaking.com/product_info.php/products_id/1006

It tastes ok.