You can purchase a new rain barrel or make one yourself. The City of Los Angeles recently piloted a Rainwater Harvesting Program and has on their blog:

a list of companies selling rain barrels
a rainwater harvesting how-to guide

How to Install a New Rain Barrel and Plan to Utilize the Water

"By observing your landscape during a rain, you can locate the existing drainage patterns on your site to your advantage. Identify low points and high points. Utilize these drainage patterns and gravity flows to move water from catchment areas to planted areas." Read more...
-from the California Urban Water Conservation Council

"How to Build and Install a Rain Barrel

Introduction

The calculations are quite easy but I have done them for you here. You can fill a typical 55 gallon barrel from an average size house with only a ¼ inch of rainfall. The following formula will help you get an idea for how much rain you can harvest from your roof. One inch of rain for every 1000 sq ft of roof surface will result in a 623 gallon water harvest. You can do your own calculation of the yield of your roof by multiplying the square footage of your roof by 623 and then dividing by 1000. You can do all of the cost justification for this mini-project but that is not the intent of this article. Some benefits of using harvested rain water include: water with no chlorine, lime of calcium, reduction in your use of treated drinking water, and the reduction in use of treatment facilities and drainage systems.

Instructions

1. Disconnect your down spout and then cut off a portion to allow a 55 gallon barrel to be placed underneath. Typically you will want to cut the down spout 12 inches above the top of the barrel but remember to include the height of any stand that the barrel will be placed on.

2. Build your rain barrel: Using 55 gal plastic barrel install a flexible elbow at the bottom of your newly trimmed down spout to divert water into the barrel. Install a water hose spigot at the bottom of the barrel to allow you to attach a garden hose for extraction of the water. And finally, install an over-flow valve at the top of the barrel. You will want to attach the overflow valve to a diverter just in case you get a big storm and a rush of water into you rain barrel.

3. Install your newly built rain barrel under the down spout, I recommend placing the barrel at an elevated position using (Two) 8” x 8” x 16” concrete blocks. Now you be able to harvest rain from your roof-top but you should limit the use of the water to outdoor irrigations, washing your car and other outdoor uses.

Materials List:
1. 55 gallon barrel , 2. ½” Hose Bib spigot , 3. ½” Overflow fitting and tubing to divert over flow water
4. two ½” lock nuts , 5. Mesh Screen and screws , 6. Silicon Caulking compound

Overview of basic steps:
1. drill opening for overflow valve (hose adapter)
2. secure with nut
3. drill hole for spigot
4. secure with nut
5. seal the over flow valve and the spigot with silicon caulk
6. cut piece of mesh to cover opening on top
7. attach mesh to top of barrel with screw"

From http://www.ehow.com/members/danarax.html