Where can I get an Apple MacBook charger US duck head?

Some of reputable suppliers of Apple accessories show items such as cables, adapters, etc.

• Apple Service Part: Apple Genuine AC Power Flip Plug for Magsafe/Magsafe2 - OWC ($2.75)

https://eshop.macsales.com/item/Apple/PWRFLIPAC/

Mfr P/N: 603-2618 | OWC SKU: APLPWRFLIPAC

(*) Apple Service Part: 1.8 Meter (72") Apple Genuine AC Power Cable - Good/Excellent Condition, 'Unused, Bulk Packaged'

https://eshop.macsales.com/item/Apple/PWRCBLMSAB/ - OWC ($5.75)

Mfr P/N: APLPWRCBLMSAB | OWC SKU: APLPWRCBLMSAB

OWC is an example: outside N.A. there are 'affiliated' resellers

Apple makes a World Travel Adapter Kit if you want all the different ones.

Or there's this Power Adapter Extension Cable.

I don't see just the single US duckhead on the online Apple store though.

You can buy the 6 ft AC extension cord version at any Apple outlet.