As you know we provide links to instructions that LEGO publishes at its customer services portal. They are available for most sets released in the last 10 years or so but coverage beyond that is patchy.
To help you to find instructions for older sets that are published on other websites, we've teamed up with new website Swooshable.com, that maintains a list of which instructions are available where, and show this information on the set details pages, under the Instructions tab. You'll now see a list of those at LEGO.com if available, and a list of other sources. Here's a good example.
Swooshable currently lists instructions at Peeron, BrickFactory, LetsBuildItAgain and BrickInstructions so hopefully it will help you track down those that have otherwise been elusive.