The 128 GB and 256 GB versions of the Galaxy S9 and Galaxy S9 Plus are now available for pre-order in the United States. These have previously been available in some select markets and are now exclusively available from Samsung's own website.
https://www.notebookcheck.net/Ultrabook-sized-storage-Samsung-gives-the-Galaxy-S9-and-S9-Plus-128-GB-and-256-GB-storage-options-in-the-US.301706.0.html