Petrol Cap
The internals of the Elise S1 petrol cap are manufactured by Stant. It's also used by Jeep, from whom you're more likely to be able to readily obtain it.
On-line sources include Rock Auto in America. The part number that you need to order is 10501, delivery should be expected in about 1 week. You may also be able to get it from a UK Jeep dealer - please add your sources here if successful.
Some info on how to swap the unit over - info taken from Basil on the BCBBS
"Best way I found was to prise the metal ring out, then vandalise all the white placcy bits which came out pretty easily. All you're left with then is the black placcy part. I ran a small screwdriver round the edge, then got hold of it with some pliers and tried to 'roll' it up. A bit of this and it popped out.
Fitting the new one I used 2 minute epoxy (time's money!) Left overnight it now works as it says it should in the manual - result.
Couple of other points, prolly stating the bleedin obvious but; Don't forget to switch the keys over on yer keyring! Don't be tempted to 'try' it in-situ before fixing. It does fit and is a bugger to get out if you're daft enough to 'try' it first. Of course I didn't do that! ;>)"