The UX is lacking and nobody knows about it, but in nostr:npub1pu3vqm4vzqpxsnhuc684dp2qaq6z69sf65yte4p39spcucv5lzmqswtfch you can search for any @user@mastodon.server and it will let you find and follow that person via the mostr.pub bridge. If you work out the nip-05 you can do it in any nostr client.

Here’s how we do it: https://github.com/planetary-social/nos/blob/a1d75348703851e4f5a51ca3ac21c26ef61882e3/Nos/Service/RelayService.swift#L734

The nip-05 for most fediverse users on nostr is: mostr.pub/.well-known/nostr.json?name=username@fediverse.server