What a beautiful safe CavalierHoo, how many places have you tried for the repair, as any good, skilled locksmiths should be able to make you another key or repair your broken one.
Its a pin safe key with the additional slot cut in the bit- a blank the right pin Gauge and some intricate cutting will do it. Even if blanks are rare in the USA then to be honest it’s not too difficult for them to make one from scratch, but it is time consuming work so will be expensive.
Another option is to modify a blank to take the remaining working end of your key, either a castellated joint or male/female with the one half turned down to a male pin,and the other drilled out female and the two mated parts silver-brazed.
Any good locksmith should be able to do this, although you might have to settle for half the stem and bow being steel.
Worth trying around the older shops first as some of the youngsters might shy away from this type of work (proper old fashioned ‘hands-on’ dirty work!)
Depending on your location I’m sure Doug Macqueen on here could do if for you, or be able to recommend someone closer to you. It’s an amazing safe and sure hope you manage to get it all sorted.