You must keep increasing your settlers and their happiness in order to maintain the growth of the Sanctuary. You also need high-level Workshop Merchants so they can sell better stuff to you which will again increase the Settler's Happiness. This Fallout 4 guide will provide you map the location of each Merchant and where they spawn.
These Locations that we are about to share are for the fixed merchants that will be available there 24*7. unlike other Locations Spots which varies on your game and Sanctuary's progress. So let's start with the Merchant Locations and how to get them to the Sanctuary.
Vault-Tec Rep (Trader)
Location: On the second floor inside one of the apartments in Rexford Hotel near Goodneighbor.
How to Get: Once you find him just ask him to travel to Sanctuary, head to Sanctuary to again meet him and offer him the Job.
Appropriate Store: Trading Emporium
Holt Combes (Trader)
Location: Inside the Vault 81 near the depot.
How to get: Talk to his wife about him and convince her to get him to the Sanctuary.
Appropriate Store: Trading Emporium
Rylee (Trader)
Location: At Longneck Lukowski's cannery
How to Get: Casually talk to her and at the end of the conversation there will an option to recruit her.
Appropriate Store: Trading Emporium
Ann Hargraves (Clothing)
Location: Inside WRVR Broadcast Station
How to Get: First rescue Rex Goodman before interacting with Anne.
Appropriate Store: Clothing Emporium
The Spot locations that we are about to share are the location where randomly any merchant can spawn. This will be a totally random encounter where you can also find a doctor or legendary merchants. Before hitting these Spots, you must be ready with Minimum 30 settlers(average needed), unlike the Fixed Location merchant which do not demand any Population need.
Spot 1:
Location: Near the Mass Fusion Containment Shed there is a parking lot where you'll see two NPC from which one can be recruited. Then Save the game, leave the zone and come back to recruit the other NPC as well.
Possible Merchant that Spawn
Spot 2:
A tricky and time consuming one, but much need merchant spawn here. That is why we will explain you Step by Step and do exactly the same in order to get this one. To begin with you must Sleep in any settlements for 3 days(72 hours) a bit far from the Med-Tek Research Center.
Note: Save/quicksave after you have slept for 72 hours
Travel to Med-Tek Research center and wait there until you see someone walking down or up the road which is obviously a random spawn. The Spawns include a caravan trader or a patrol and if you are lucky enough then a Legendary merchant will spawn here. The Trick here is that if you don't want this particular merchant or any other person then Quickly load the last save instead of waiting 72 hours.
Possible Merchant that Spawn
If you are unable to recruit this merchant then the reason will be the population demand where you must have enough settlers. Try to quicksave within the same range of the spot so if that person didn't appear the same exact spot then you can reload your last save and then hit the spot till that person is there.