You'll find plenty of links to sites with custom hatch patterns enough to fulfill your wildest dreams. One last thought, search the Internet for 'AutoCAD' + 'hatch patterns'.
Otherwise, any good reference book about AutoCAD should have at least one chapter dedicated to customization, including linetypes, hatch patterns, command aliases, Lisp programming, etc. If you do a search of this site you'll find further information about it. Depending on which version of AutoCAD you are running you might want to check out the Superhatch command.
