There is currently no legal way to purchase Apple Watch Series 9 and Apple Watch Ultra 2 (models with pulse oximeters) in the U.S. Apple immediately appealed the ITC's decision. https://www.notebookcheck.net/Apple-Watch-U-S-import-ban-is-enforced-today.787448.0.html
They also will not be able to perform hardware repairs on Apple Watch Series 6/7/8 until the ban is lifted.