If you construct a Trade Caravan Post in one of your settlements, you'll find that it is frequently visited by one of the several roaming vendors normally found out and about in the Commonwealth. Depending on what you're looking for in a "Trashcan Carla replacement" these may or may not fully suit your needs. But the post is handy to have regardless.
If you do not have the Trade Caravan Post available for construction already, make your way to Bunker Hill and complete a few quests there. It's probably best to stay friendly with those folks, too. I don't know the exact trigger, and the Internet seems uncertain as well, but most sources I've found indicate that one of the Bunker Hill quests (nobody else seems to know which, either) should unlock the item.