This video will show you the locations of 3 legendary merchants, these will be gaurented to spawn here so make sure you get them, for Ann Hargraves you have.
When I was looking into it myself I could never find any real solid information on how to get them. As a note, this requires the use of console commands, so if you refuse to use them, or are reading this looking for advice while playing on Console it sadly will not help you. 3:45pm How to easily get any Tier 4 Merchant.
Rich Merchants at Fallout 4 Nexus - Mods and community Fallout 4 > General Discussions > Topic Details.
He can be found in the back of the Bait shop in Far Harbor There is a doctor who will sell you a shipment of 25 Antiseptic for 705 (might be higher or lower depending on your barter skill).
Expert merchants are special characters who can be recruited for a settlement to man each type of store. What follows is a list of merchants and the items they can sell.