An easy way is to go to someone’s profile and click on there relay list - you can then add them from there (at least in #damus you can)
Or you can use these Websites with lists of relays and their performance/health:
relay.nostr.net - relay run by nostr.net
nostr.info - real-time checking of status of some known relays
nostr.watch - real-time checking of status of some known relays