Yup, pretty random ... they'll show up eventually. If there's a store you haven't been to at all yet (haven't even talked to the merchant) you can save your game, and if they don't have any robes, reload and try again. Some of them change their inventory, within limits, if you do that. Once you've spoken to them first tho, it's set.
Just to mention - some of the better stat-robes, imo, are a lot uglier. I couldn't stand most of them, so I often went thru the whole game using just the black Padawan robe, because I'm character-clothes-snob and they looked the best on-character. On the female models, anyway.