Can I digitally buy and download the first Diablo game?

Solution 1:

The original Diablo game is now available digitaly on GOG, DRM-free and supported on Windows 7, 8, and 10.

https://www.gog.com/game/diablo

You can try searching on Amazon for either the PC or PlayStation physical copy. If not, look for other releases or bundle. In 1998 there was a re-release bundle called Diablo + Hellfire. Blizzard also released a Game of the Year Collection in 1998 that contains the original Diablo. The Blizzard Anthology, released in 2000, also contains the original Diablo game. The Diablo Gift Pack contained the first two Diablo games but no expansions. In 2001, a Diablo: Battle Chest was available which contained the original Diablo, Diablo II, and the Lord of Destruction expansion.

Diablo II, however, is still available from Blizzard's online store. The original Diablo II game, Lord of Destruction expansion, and the Diablo II Battle Chest (which includes the Diablo II, its expansion, and strategy guides for each) are all there. However, the Battle Chest for Diablo II does not include the original Diablo game, but Diablo II and its expansion are available for digital download.

Solution 2:

Situation changed just a few days ago. gog.com offers it now. https://www.gog.com/game/diablo

Solution 3:

The Diablo 2 Battle Chest comes with Diablo 1 game.