you should start collecting urls:
- get a server up now and monitor and analyze all 404s.
- start looking for any cached or scraped pages and see what you can find.
- get some accounts going to use the various inbound link discovery tools <cough>majestic</cough> and add that to the mix.
- provide a forum for your members to supply anything they know of through bookmarks or their own content that links to your site.