Unauthorized dealer Tristate via eBay has the Canon Extender EF 1.4x III (grey market) for $363.97. Compared to the history, this is a very low price for a brand new EF 1.4x III Extender, but this is grey market so a Canon USA warranty is not guaranteed.
Extender EF 1.4x III for $363 Brand New (Grey Market) via eBay
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And you will need a warranty because these are famous for a sticky locking button.