There's not a lot of Magic: The Gathering cards that you couldn't theoretically get ahold of. Even cards that were printed a grand total of once, like Shichifukujin Dragon, can be theoretically owned. Back in 1997, however, there was a set of 12 cards that could only be found in video game form.
The game itself was Micropose's Magic: The Gathering computer game for Windows '95. We could go on all day about it, and as a matter of fact, we have a video all about it on YouTube. While the computer game boasted cards from many of MTG's earliest sets, including limited, Arabian Nights, and so on, it also featured the game's digital-only Astral set.
Take a look:
The Micropose's game cards are officially referred to as a "mini-expansion". A special card symbol of a shooting star was even added in to make it more in line (visually) with the game's physical expansions. Really, Wizards of the Coast and MicroProse went pretty much all out on making these as true to the game as possible while also taking into account the advantages of working within a digital space can afford.
And when we said that Magic went all-in to make these like as real cards as possible with artwork, we weren't kidding. Liz Danforth, Quinton Hoover, NeNe Thomas, Amy Weber, and others a handful of all put in work. It's really too bad that none of the cards ever actually saw print.
Well, except for one.
As a promotion, 50,000 oversized copies of the card Aswan Jaguar were included in the first 50,000 game boxes. Of course, being oversized, it was neither allowed nor even practical to allow it to be played in any of the game's various formats. As such, it's only worth maybe $10 or so.
While digitally-exclusive cards have come out here and there since the MicroProse PC game (and more so as of late in Magic: The Gathering - Arena), the cards found in the game's Astral set do mostly live on through stills and other digital forms today, as well as the oversize promo card print run, the odds that these cards would ever see print as a whole remains a nigh-impossibility.
So, enjoy them digitally. It's probably the closest any Magic player will get to literally getting their hands on any.