I'd say it was mid-June that the fiatusa.com build system went live... The first 124s hit dealer lots in Late June.
The top fiat dealers in the US received the first 124s the first week of July, possibly late June.
I got a call from my local dealer yeaterday and they said they placed an order last week for a new car and it came back as being a 2018. it was the first customer order they placed that outputted a 2018 model. That said, I don;t think I will be ordering my car until the online configurator is avaialible. No major rush.