Jim, the original Lucas mirror was used on Jaguars, Aston Martins and Austin Healeys of the period, some still available if you look hard enough, keep checking E bay as I have found several there, Frank
I just did a quick search on eBay, and found one that's in the general ballpark. They do come up all the time so if you're not too happy with the price or quality, no worries... look again next week!
I've found that a inside rear mounted camera works better than a mirror - even with the best mirror the view through the rear window in most GT40's is badly obstructed.
If you're interested, I'll send along some pictures of my solution although there is an existing thread that examines this issue!