Although the interwebs are filled with sites from clubs and Lotus Elan owners like myself, IF you are a new owner I would personally recommend visiting the following sites.
Another very well respected club that has been around for over 40 years. Not strictly a M100 site but they offer news of local events, Lotus related news and parts cross references for all lotus cars, including the M100 (which comes in very handy)!
Hosts the largest database of Elan M100′s in the US. If you are thinking of buying an Elan in the US then checking out this site is a must! If you are in the US and have any questions related to the M100 then I would recommend contacting the owner of this site. John is as an active member of the LEC (Lotus Elan Central) and the registrar for the M100 in North America.
Of course do not forget the Home of Lotus. www.lotuscars.com Where you can find all of the latest news, brochures and tweets that Lotus has to offer.