Rain barrels can be constructed from any food-grade 55 gallon drum. Plastic barrels are easier to work with and last longer than metal barrels. In the SF bay area unfitted (no hose attachments) you can get free barrels from Keith Wilson (firstname.lastname@example.org, 805-680-3666) in Richmond. (These are closed top, food grade plastic, either white or blue.)You can often find plastic food grade drums from breweries, bakeries, or food import stores.
Plastic rain barrels can be purchased new with hose fittings from many hardware stores. These usually cost over $100. There are also on-line rain barrel sellers, but shipping costs are high (not to mention the environmental impact!) so buying local is preferred.