There are not any imported trainsets in use by Amtrak. "Buy American" restrictions in the grants require US assembly. However, there being no more US passenger rail car builders, it is done in US plants of US subsidiaries of foreign companies. Bombardier (rail division taken over by Alstom, IIRC, so formerly Canadian, now French) at Barre, VT, CAF (Spanish and the builders of the V IIs) at Elmira, NY and Siemens (German) at Sacramento, CA.
Since all of Amtrak's car suppliers are under the same basic model, US subsidiary of European car builder, parts availability or lack thereof is more related to the vendors themselves, CAF being pretty much a disaster and Siemens apparently OK, and Amtrak's contracting and vendor management.