Sometimes if it's been caked on there for a while it doesn't work 100%. And sometimes it can leave a little color staining. But it's a lot better than having big old streaks through the middle of the board.
You could probably make them yourself if you have dry-erase markers that work on glass and have access to a color printer. But if you Google "dry erase meme" or similar, you should be able to find that picture and go to that company's page.
Kids today will never know the rhythmic tapping of chalk on slate, or the screeching of nails clawing their way down, banging the erasers together to clean or watching the teacher struggle to get as many uses out of a piece as possible. Mmmmmmmmmmm