Food grade drums that were used to carry juices, olives, pickles, etc. are usually used as the barrel. Local recycling centers and bottling companies are good places to find food grade drums.
Avoid using rain water that has been collected from asphalt roofing, redwood, cedar or treated wood shingles on food-producing gardens. The aforementioned surfaces may leach toxic material when wet.
Make sure you thoroughly clean out the food grade drum before using it. To clean the barrel, rinse the inside of the barrel with a mixture of 1/8 cup of bleach and 5 gallons of water to wash away food or juice remnants. An environmentally friendly way to clean the barrel is use 2 tsp. of castile soap and 2 tsp. of vinegar or lemon juice for every gallon of water used.
Click here for instructions on how to build your own rain barrel.