@mobanks : That will be tough, what the normal process is, is that the initiator of the order would receive an email shortly after delivery of the Premium package that includes the code. If that email was not received for whatever reason then it is possible to make a request from support for the code again but that requires both the original order number and the serial number from the Chromecast Ultra.
I would probably just suggest contacting support directly if you are unable to obtain the original order number and take if from there and see what their suggestions are.
@mobanks Just to add, Google is in no way obligated to provide codes for Stadia kits purchased through unauthorized third-party sellers. And Google only currently provides codes for bundles (controller + Chromecast + Stadia Pro), not for individual controllers; and, again, that's only for bundles purchased through the Google Store.
Eventually you should be able to purchase a Stadia Pro subscription without it being bundled with hardware (and you'll also be able to sign up for the free tier as well) - but we're not there yet.
So if you're looking to play Stadia in the near future, I'd recommend purchasing the bundle directly from Google rather than through a third party.