Photo printers are thankfully a lot less expensive than a few years ago, and having an instant photo available for your photo booth makes such a difference. Remember to buy all necessary cables you need if you're hooking up your camera to a printer. Alternatively, go for a wireless one.
If you're using your phone, then a portable printer like the Kiipix printer or the more expensive HP Sprocket is ideal. They print out small Polaroid-style pictures in really great quality. The Sprocket also has image editing via its app, which means you can add your own hashtags on the image or even filters.
Bonus tip: if you're using a printer that relies on WiFi, make sure you check with your venue that you can access the network where your photo booth is. Also, set up all WiFi passwords and logins before the event begins.