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
Years ago (maybe still) you can buy them in-person at the Bureau of Engraving and printing in DC. I have a sheet of $2 bills.