You just need to get hold of old keys, CD-Version. They don't need to be registered, they don't require all that anti-consumer/spyware/DRM crap, that sadly became industry standard.
You can play and install it from Iso-files, once. And then just play the game using the installed folder, it doesn't even require an installation to run. In the past this CD-copy protection checked the CD in drive for authenticity, but Blizzard thankfully removed that check in one of their patches, when CDs became obsolete. It's all legit.
Back in the file-sharing days you could download the d2 folder from some random unfortunate soul, who shared his C: drive, and play with it on a different realm, where it wasn't in use. A lot of people had diablo installed back then. Pity i haven't kept them.