https://lemmyverse.net/ is the most complete list you’re likely to find since it uses a crawler to find federated instances and their communities from an initial seed list of known instances. So unless an instance is defederated from all of the instances in their seed list then it should appear on there. It also counts subscriber numbers, etc. as a sum across all the instances it finds, which should give a sense of where the majority of the community are gathered if there are duplicates.