You can buy many different styles of rain barrels or make your own. They typically range from about 30 to 50 U.S. gallons in size, but can be linked together to hold more volume, such as my home rain barrels, shown here. I made them from former soda syrup barrels. My wife uses them to water our flowers and I sometimes connect a drip irrigation hose to them to water nearby trees and shrubs.
A wide variety of rain barrels and rain barrel parts are now available for purchase over the Internet and occasionally from local retailers. Building a rain barrel is not terribly complicated if you like doing projects yourself. To help you get started, attached below for download is a paper describing the steps for building your own rain barrel.
If you can find a source of barrels in your area, all the associated parts needed to turn it into a rain barrel are available from your local hardware store. Plastic barrels are typically obtained from the food industry. The ones I have used previously held soda syrup or pickles. Soda bottling plants seem to be a common source for used barrels. Non-profit environmental organizations, groups involved in watershed protection, or businesses that repurpose used building materials tend to be sources for barrels, as they often obtain them in bulk and resell them for workshops and fund raisers.
When you buy a used barrel or get one through a workshop, ask about the source of the barrel from the supplier, look at the labels on the barrel, and look inside to see what they formerly contained. I once came across a barrel at a workshop that came from an industrial chemical plant and had adhesive residue in it. It is best to only use barrels that were previously food-grade.
I hope the attached How-To paper is a helpful. But if your first rain barrel is a flop, just stack it along side your house like this place and use it for shelving. Happy rain harvesting.