Most of the ActivityPub-based networks have a "join<service>.org" index of instances --

https://joinmastodon.org/

https://joinpeertube.org/

Hopefully this is helpful!

Note that there's far more instances than https://joinmastodon.org/ suggests.

Theres a directory literally on the front page of the "main site": https://joinmastodon.org/

Beside the official currated one you have: https://fediverse.network/ https://instances.social/list/advanced https://www.fediverse.space/