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
Thank you, friend. I really needed the visual of going to the bank and making a down payment on a house with some big fucking currency sheet that are completely uncut. I am forever in your debt.