Where do I find my 26 character CD key?
Solution 1:
You can log in to battle net and go to
https://us.battle.net/account/management/
(eu or us shouldn't matter)
Click on Diablo II, you'll be taken to the page with the key(screenshot is in Italian, but that shouldn't be a problem):
Solution 2:
The key you want can be found under the details for that game in your account info on the battle.net website.
battle.net -> log in -> ACCOUNT (at the top) -> Diablo II
The GAME KEY will be visible on this page, as well as links to download the game.