You never heard of Steam? It's Valve's (the creator of Half-Life) digital video game distribution service that most people use nowadays to market their games and buy them. KSP was put on Steam back in (I wanna say) 2013-14. It goes on sale all the time.
https://store.steampowered.com/app/220200/Kerbal_Space_Program/
I recommend just buying the base game as the expansions are more-so niceties that you can add freely with mods. The expansions are cool, but no worth spending money on if you
just wanna play the game. The modding community; albeit slowed down a bit in recent years, is still bursting with great quality, free content.
