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Jiggers
If you want to make a good cocktail, you need to measure it out. Even the best mixologists in the country use a measuring tool, a jigger, to make sure they get their ratios right. You can get a stainless steel jigger for less than $10. It should be able to measure up to 2 oz. A graduated one will be the most versatile for any recipe. A quick search online will find a lot of options to choose from.