You probably do not want to use regular cow's milk with this routine, as milk can spoil and because of the fat content, can be difficult to rinse out of the Milk Glass. Here are some other options:
Option 1: Use soy milk, which does not spoil and can be sipped during the routine to prove that it's real milk.
Option 2: Mix up some powdered milk just before showtime. Again, the milk can be sipped, and it is easy to transport because you keep it in powdered form until you use it.
Option 3: NOT RECOMMENDED to use the solution marketed as OOM. This is concentrated machinist's cutting oil which, when mixed with water, turns the water a milky color. It's not suitable to be ingested! It also has a strong smell; your audience will know it's not real milk, and will mistakenly think that the trick works by using some sort of fake milk. Plus the oil will cause your Milk glass to discolor over time. Not good!