This addon will alert you when a rare spawn is nearby.
I couldn't find a classic version of NPCscan, so I expanded on the existing "unitscan" to mimic it's behavior.
I do not claim any ownership of unitscan or NPCscan.
Note - The list of rares/locations originally came from Classic Wowhead, and it is not perfect. Some rares may be missing, and some "rares" may be listed by mistake. As such, you can add/remove from the list via /unitscan name. Happy hunting!
Commands:
/unitscan lists the active scan targets (entered by the player).
/unitscan name adds/removes name to/from the scan targets.
/unitscan rares lists all rare spawns and their locations.
/unitscan nearby lists the rare spawns in the same zone as the player.