If you’re looking for private WOW servers you should check out server lists, server lists that allow players to vote for their private WOW servers. Why? Mainly because that’s the one thing that guarantee’s that the server is actually populated.
Private WOW servers: https://serverlist101.com/wow-servers/
There are still no questions asked go to private WOW servers, for example Legion server would be one of those, WotLK server and Warmane private WOW server. If you’re looking for something new, something different then you’re going to have to do some research and check out different private WOW servers.
Also consider this, not all private WOW servers are running on the same version, some WOW servers are using older version with expansion packs and newer graphics, while others are using up to date client with latest content found on official WOW servers.
The choice is yours.