Use TD
Sorry, it seems like a US AMEX isn't going to be an easy get. AMEX US will only use history with AMEX Canada to issue you a new card.
In the meantime, I suggest getting a card issued by the US arm of a Canadian Bank. I've gotten a Visa card from TD USA and they did a Transunion pull of my Canadian credit report to issue it. It was pretty easy, just call cross-border banking support.
I don't have experience with the other banks but BMO and RBC both have US affiliate bank which should be able to get you a card to build US credit history.