The letters come individually. I believe I linked the company that sells them above and mentioned which letters I ordered. If not, here's a link to the very page with these letters. If you type in the blanks at the top, it gives you a preview of how they will look.
Chrome Letters and Numbers - Domed Small | No Minimum Order
You can choose different letters if you like. There are different fonts. But be careful about the size. I almost ordered some taller letters which would have been a big mistake. Luckily, copied them from the site, sized them to life size, cut them out and checked how they would look. Even the letters that were just slightly larger were MUCH larger when strung together and some wouldn't even fit lengthwise on the case!
BTW, you really don't need to make a cardboard window, of course. Just a "L" shaped piece of cardboard is fine. Tape them to the cases. All it really does is line up the first letter on the left and then keep all of the letters at the exact same level on the bottom. This isn't rocket science. You just order the numbers and letters, figure out where you want them, device something to tape on each case as a guide (I used cardboard) in the same place on each side, and stick them on. Nothing to it.