luckduck you can find the non-Steam version when clicking on Play/Download Zero-K on this website.
Oh, and btw: you cannot use your Steam account with the non-Steam version, you have to create a new profile. It will ask you to do so once you open the game. It is possible to write some admins (these guys with the symbol of a head in uniform before their nick on this forum) and tell them you want your Zero-K account linked to Steam transfered away from Steam and use it separately. I did not do so yet, and sometimes, when I open Zero-K and have Steam process running in the background (beware, if you close the window it does not mean you closed the app, you either have to change it the settings or close the process maually in task manager), it logs me in with the Steam profile, even though I dont use the Steam version. Then, you just have to close Zero-K, close the background Steam process, and open Zero-K again.