If you live in a region where rainwater harvesting is fairly common (U.S. Virgin Islands, parts of Texas, Oregon, & Arizona, etc.) you should be able to find system installers in the yellow pages. In areas where rainwater harvesting is not practiced frequently it may be more challenging to locate equipment and installers. Roofing and gutter installation companies may be able to help you locate rainwater harvest system installers.
A substantial amount of information is available on the internet including sample designs and a list of suppliers. Rain barrels are available from a wide variety of suppliers on the internet or at builders hardware and gardening stores. The following web sites should be useful in locating sources for rainwater harvesting systems: