Hi Dave,
The Golden Gate list which Joe has posted is the most comprehensive I've seen in terms of the manufacturer's part references and sometimes the original donor car. The trouble these days is that a lot of those donor cars aren't around, or if they are they are now classics in their own right and no longer a cheap source for parts.
Like most owners I've got a well thumbed book where I've noted parts when someone's told me or I've stumbled across something at autojumbles, so if there's anything in particular it would be wort a post to see if anyone knows of an alternative source. But since Ebay has grown in prominence, there's not much chance of finding that pair of door handles as NOS for a fiver at an autojumble I'm afraid. (Although I did pick up a brand new gear lever gaiter for £2+postage on Ebay last year - it was a Rover part IIRC)
If it's a case of "what's missing" that's the problem, then of course there's the Lotus parts manual (pdf format) which is out there, plus the main suppliers such as SJS, Banks Europa, RD Enterprises, etc, who usually manage to come up with most things between them and often have pictures on their websites so you can see exactly what you're getting.
Brian