Rainbow tables help with the discovery of passwords by speeding up testing time. You could compute your own, purchase pre-computed tables, or download tables that are made freely available to everyone.
Recently a new set of tables was published for free public access. The Hak5 Rainbow Tables consist of about 120GB of pre-computed LanManager passwords. If you decide to use them be aware of the parameters used to build the tables, which include the limitation of password lengths of up to 7 characters.
You can learn more about the tables at the related blog where you'll also find a link to a Torrent download.