Shes only available on certain rotating days(one out of 5 days) and doesn't always spawn in your own garrison.
They stand right inside the front door of your main garrison building. (most of the time there will be some random dude like Harrison Jones or an SI:7 with a blue exclamation mark standing in his place)
You can check in premade-custom groups for other people hosting a party for whichever trader is available on that day, and you can view their garrison and go to the trader.
This post was edited by cambovenzi on May 14 2015 10:51pm