Private servers are typically advertised on (e.g., TOP100, GTop100) or dedicated MMORPG private server forums. Searching "Zero Online private server" yields various results, but due to their nature, links change frequently. It is recommended to:

: You must modify the config.ini or equivalent file in the Zero Online game client so it points to your private server's IP rather than the official servers. 🌐 Finding Community Support

As servers open/close frequently, do a live search. But these are the usual categories: