Your best chance is contact Smarts R Us or MisterDotCom who currently sell them.
They are still available elsewhere, they were called "Easycap" and were universal fitments.
The cap screws into place where the original one should go, it has a hinged flap in the middle so the petrol nozzle can be pushed straight in without the need to remove the cap.
It does make filling a little harder as you have to jab the nozzle in with a bit of force otherwise the petrol pump refuses to deliver petrol.
Once it's in it's a bugger to get back out but nothing a twist and pull doesn't sort out.
Ultimately it saves time and effort undoing the original cap.