Official KSP legacy builds are sometimes archived on the official forum's "History" section. Be wary of third-party "abandonware" sites; always scan for viruses. Better yet, if you own the current KSP on the Private Division store, you might have access to the legacy launcher.
Kerbal Space Program 0.15 is the Model T Ford of space sims. It’s slow, dangerous, and lacks seatbelts—but it started the journey that landed us on the Mün. KSP - Kerbal Space Program - 0.15 Download
But 0.15 has something modern KSP sometimes loses: Every launch felt like a real test flight. Official KSP legacy builds are sometimes archived on