There are many form books available online and in print. A few of the major titles for federal and California forms are listed below, but there are many more including for other states and focusing on specific practice areas. Your favorite treatise in a topic area may also include some forms.
Some of the major legal research databases also have form finders to help you search for forms across publications by state or practice area. Links to the Westlaw and Lexis+ form finders are on the homepage of each database.