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
adding “wallpaper my house with these” to the list of things I would do if I had infinite money
Just be careful with the wallpaper paste. Mutilating legal tender is a federal offense, so your interior decorator will need a security clearance.