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any figure eight brace will work. like this one or similar, any brace that pulls your shoulders back and can be fixed properly. but you'll need someone to help put it on you and tighten it, it's pretty much impossible to do it correctly by yourself.
i used to slouch quite badly and then i broke my collarbone. had to wear one of these things for a month non-stop, and it was extremely uncomfortable the entire time so you really should question whether it's worth it for you, but i don't slouch anymore, that's for sure.