The Bureau of Engraving and Printing sells them through the official government website, priced above face value as collectibles. Every note can be cut out and spent. Handing the whole sheet to a cashier is also valid, though they will silently curse you.

Credit: JFitz717

https://www.usmint.gov/paper-currency/uncut-currency/?srsltid=AfmBOoq8fxAfYbLBCpgkeDTXzHpea3etlIRJqI7h1gyh-iX80WrZaTi_