Serious Eats: Recipes
Amaro is just a general name for a bitter, herbal liqueur traditionally served after a meal.
Gentian root can often be found at Latin markets and specialty herb shops. You can also order it online from companies such as Lhasa Karnak. Use the cut root, which looks like bits of bark, rather than the powdered variety. If you have another bittering agent on hand, such as cinchona bark leftover from making tonic, you can use that as a substitute.
If you can't get a high-proof neutral grain spirit like Everclear, use the highest proof vodka you can find.