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):

Diablo key

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.