There are semi-silvered mirrors that can be used for this purpose, and plain glass will also do. I used one in an experimental self-portrait setup - exactly as in a photo booth (they usually use plain glass). You need a reflecting surface at least half the size of the area you want to see (that's a general rule for flat mirrors, of course).
You can do it with a plain piece of glass, if you can black out behind the glass and around the lens. That is not difficult.
Get a good quality piece of plate glass and fix up some way of holding it directly in front of the camera - you can hold it against the front of the fully extended lens to line up the lens and the glass. Clip black velvet or black Cinefoil (heavy aluminum foil with a black coating) or similar behind the glass and around the lens so that no stray light can get between the glass and the lens, or through the 'back' or edges of the glass. A rubber lenshood is useful for the lens-glass connection.
The plain glass will give you enough of a reflection to see your expression and, if you set it up correctly, the composition of the photo.
If you've never seen how it is done in an auto photo booth, it is worth finding one and checking it out.
Best,
Helen