If one of your keycaps is broken, and you’d rather buy a replacement than use one from another key, you might be able to find a new one on sites like. Obviously, the label on the key will be incorrect, but you can relabel it with a write-on sticker if you want.
Once you get the keycap off, place it on the switch of the missing key and push down gently until it snaps into place. Just be careful not to use too much force-they’re easy to break if you slip or pry too hard. You can usually gently pry off a keycap with a flat head screwdriver. Again, you’ll want to choose one you don’t use often. If a keycap breaks or goes missing, but the switch beneath it is okay, you can potentially use another similarly sized key’s cap.