Low Fees Buddy on Nostr: Search the node name in amboss. From there you can get their telegram, twitter, nostr ...

Search the node name in amboss. From there you can get their telegram, twitter, nostr or website in some cases.

If not, search the node name in LightningNetwork.Plus if he participated in swaps etc. he'll have an account so you can message from there.

If nothing else works, try a keysend, but they'll need to have it enabled in the conf.