Pyros
<Silver Donator>
Maybe a bit later than 15 but not too far, Hunter's Edge guy just outside is basically first thing you see in the zone and stocks a lot of legendaries, homestead eventually has a vendor that stocks a decent amount too. Just doing these 2 every level is fine to gear up almost entirely in legendary. Can even save scum if you really want a specific upgrade. While questing there's a few other vendors that stock legendaries too, spoilered since actually spoilers(act1 and 2 spoilers though):
Jack sells legendaries and you can make him spawn until you progress the story by clicking his grave in town after you've done the Evelyn shit and the Fire elem guy from the frost zone also sells shit, though I don't know if you can make him respawn but you can reload a few times to get a nice legendary there since he sells 2-3 different ones