Any quality, high proof alcohol may be used, but many herbalists, myself included, prefer a neutral spirit like vodka so the taste of the herb comes through.
Most tinctures use ethyl alcohol that can be found in high-proof spirits such as vodka or brandy. These are commercially available and very safe for consumption.
Remember that regardless of the alcohol chosen, it has to be at least 80-proof (namely, 40 percent alcohol) to prevent any mildewing of the plant material in the bottle.