ATR Magnetics makes and sells cassettes here in the US. They are selling off their Ampex ATR service division- there is an announcement about that on their home page- don't be put off by it.
Blank audio cassettes ARE still made, even in the United States
I bought some bought haven't tried them yet. Need to get the cassette deck worked on yet.
National Audio in Springfield Missouri is the manufacturer.
They have three types of blank cassettes.