A while back people were talking about using a DHT to distribute relay lists, and you could use that to lookup where someone posts to. But it didn't get much traction.
When I initially wrote NIP-65 I had the epiphany that since these relay lists are small, don't need moderation, and are replaceable, it's not much overhead for relays to hold tens or hundreds of thousands of them (compared to the number of events that they hold), ... at least at this stage ... and so it recommends just spamming your relay list event all over the place. If people are actually doing that, you can find their relay list just about anywhere! But that solution probably won't work as the size of the community grows and as relay usage gets more and more decentralized.
Some relays advertise themselves as directory services, such as wss://purplepag.es/
So we're good for the moment I think.