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
Sounds like something fun/evil to do to fuck with people, give them slightly ill dimensioned, but completely legit bills.
Forever trying to square away the money in the cash registers.