[Conscience of a Conservative]

Some pretty reputable quote sources have it as Palin tweeted

I wouldn't be surprised if Palin misquoted it by copying it from one of those sources - perhaps she couldn't recall the exact quote (but knew that Plato said something along those lines), and googled it to confirm (finding the incorrect, but commonly-misquoted "quote")

[Al B.]

She also referenced something Plato said many months ago in an interview with Hannity.