Honestly, it's not suuuuper bad if you limit robe wearing to Diagon Alley/shady spots in Hogsmeade. I pack mine and shove it back in my bag when I leave WWoHP areas.
I don't think there are any rules of where you get your robe. I am doubtful you would find one in a thrift store, but it certainly wouldn't hurt to look.
We went the 4th to the 11th and saw plenty of people in robes, including a father and his children all in robes. My son wanted one, but we said no that he would be too hot walking around in a robe. We weren't spending 100 dollars either on it
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