Is there a way to legally obtain a copy of Minecraft to join "offline" servers with?

I host my own Minecraft server and have online-mode set to false in serer.properties.

Now, I want others to join but I don't expect them to buy a legitimate copy of Minecraft. Isn't that why we use this setting online-mode?

In the ol' days (1.8 and post), I remember using some copy of Minecraft where I could just change my username on every start of this client, and join my server. Period. Done. I want the same again. Is this possible? In a non-illegal way? What about grey-area? I don't care about security or impersonation (partly).