Purchase Diablo 3 FAQ.

Where to purchase Diablo 3?
http://www.nzgameshop.com/pc-games/diablo-iii-3-game-pc-and-mac
You can get it from here, price is pretty decent, and store can be trust.
http://www.g2play.net/store/Diablo-3-US-Digital-Download.html
This website price is little bit higher, but you don't have to wait, they will send you Key on day one. Store purchase game from third world country and resell, some game company ban for that action. Purchase with your own risk.

Edit:
Do not purchase 1499NT Taiwan traditional Chinese D3. Else you will be lock into single language D3 pack.

Then where to download English version?
Here you are. Official download link:

enGB: http://us.media.battle.net.edgesuite.net/downloads/d3-installers/589acdb4-c660-318a-b993-01528fad7da6/Diablo-III-8370-enGB-Installer-downloader.exe
enUS: http://us.media.battle.net.edgesuite.net/downloads/d3-installers/589acdb4-c660-318a-b993-01528fad7da6/Diablo-III-8370-enUS-Installer-downloader.exe
enSG: http://eu.media.battle.net.edgesuite.net/downloads/d3-installers/589acdb4-c660-318a-b993-01528fad7da6/Diablo-III-8370-enSG-Installer-downloader.exe
Traditional Chinese: http://eu.media.battle.net.edgesuite.net/downloads/d3-installers/589acdb4-c660-318a-b993-01528fad7da6/Diablo-III-8370-zhTW-Installer-downloader.exe
Credit to people from reddit.