I purchase my Arrowroot Powder here: LINK
To store Arrowroot, keep in an airtight container marked with the date that you bought it. USE WITHIN 2 MONTHS because its thickening properties diminish with age.
FOR ONE CUP OF THICKENED LIQUID: 1½ teaspoons of Arrowroot in 1 tablespoon cold liquid. Stir or whisk the cold mixture into the cup of hot liquid at the END of the cooking time. Stir until thickened which is about 5 seconds. Stir until just combined, over-stirring can make it thin again.
CORNSTARCH REPLACEMENT: Replace 2 teaspoons of cornstarch with 1 tablespoon of Arrowroot powder. (FYI: 3 teaspoons = 1 Tablespoon)
FLOUR REPLACEMENT: USE HALF AS MUCH ARROWROOT AS FLOUR.