First, check the Lisp Code Directory to find the name of the package you are looking for. (See section How do I find a GNU Emacs Lisp package that does XXX?). Then check local archives and the Emacs Lisp Archive to find a copy of the relevant files. Then, if you still haven't found it, you can send e-mail to the author asking for a copy.
You can access the Emacs Lisp Archive via anonymous FTP:
/archive.cis.ohio-state.edu:/pub/gnu/emacs/elisp-archive/
Fetch the file README first.
NOTE: The archive maintainers do not have time to answer individual requests for packages or the list of packages in the archive. If you cannot use FTP or UUCP to access the archive yourself, try to find a friend who can, but please don't ask the maintainers.
NOTE: Any files with names ending in `.Z' are compressed, and you should use `binary' mode in FTP to retrieve them. You should also use binary mode whenever you retrieve any files with names ending in `.elc'.