A lot of private game server lists like https://serverlist101.com/ list Minecraft servers and Skyblock servers in specific categories. Not all Skyblock servers are the same tho, some servers are unstable or lacking features and then there are a few servers that are actually good.
So how would one find good Skyblock servers? Server lists are good start but you also gotta check out servers themselves and their reviews. Good skyblock servers usually have a few starter islands not just one and generally those islands are custom, not the ones found in default skyblock plugin.
Challenges are also big part of skyblock servers, but some skyblock servers have challenges disabled, usually that’s the first sign skyblock server doesn’t have many features or it’s not updated.
Then there’s economy, most of skyblock servers nowadays run spawner based economy, but there are a few servers that use oregenerators and run block based or plant based economy, you can’t really tell if server is good or bad just by looking at it’s economy, but I guess it is something to look for if you don’t enjoy planting seeds and farming day after day.
So there you have it, hope you find yourself a good skyblock server!