To make your magic mirror, you need a two-way mirror. The most popular option is getting a plastic mirror, most often made from plexiglass. It’s a cheap option that’s easy to find online and local stores. Another option is getting a real glass two way mirror. This is a little bit more expensive, but if you can afford it, it does look a little better than the plastic ones. The guys at Twowaymirrors.com are professionals and offer incredibly high quality glass that is suitable for magic mirror making. I recommend their VanityVision glass.
The third option is using a mirror film on normal glass. Many people have been buying this film, with good results. But I recommend using it on glass instead of plastic, since the plastic tends to warp, creating a “fun-house” kind of effect on the mirror.