Garlic Juice is sold in stores in small jars or spray bottles.
You can get cold pressed, or juice from roasted garlic cloves.
To make your own, purée cloves of garlic, then press and strain them. Use immediately. It is not safe to store for long periods.
Commercially-purchased Garlic Juice will have preservatives added to it, such as citric acid and sodium bisulfite, to keep it safe.
Equivalents
3 large cloves garlic, peeled = 1 tablespoon of garlic juice
8 sprays of garlic juice = 1 teaspoon garlic juice = 1 clove
1/8 teaspoon garlic powder = 1/2 teaspoon garlic juice