You buy the game from GOG.com.
https://www.gog.com/game/sid_meiers_alpha_centauri
Then you download the Loki installer.
https://www.liflg.org/?catid=7&gameid=90
I actually bought it from tuxgames back in the day. You can take my word that the package was the regular Windows version in its normal box, plus a separate disk containing just the Linux installer.
And no, I don't believe I have either anymore. I think they were both in the box of games I gave to charity about a year ago.