Go to autozone/pep boys, look at the caps and see if they have one that will match it.. its basically an o-ring right? Or you can try this (unsure of application) http://www.ebay.com/itm/Ferrari-360-430-328-355-Mondial-Many-Others-Gas-Cap-Gasket-Seal/401283166548?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D2%26asc%3D40130%26meid%3Dcf1bd21db4804c099ae85703c6004d46%26pid%3D100005%26rk%3D1%26rkt%3D1%26sd%3D371927101210
You can check ricambi to see if they sell just the gasket. I would guess it is the whole cap assembly, but have not checked. Assuming that is correct, I would get a digital caliper and measure. Then go on mcmaster-carr and find a replacement. Good luck.
They sell them separately for the 575M, but not the 360 from what I can see. Not sure if they are similar or not.
I have one. I bought a new cap entirely so I have the old cap and gasket. I was keeping it as a spare but if you need it, let me know.
Now that's what I love to see here. From what I remember, it's not the first time Sherpa has offered something like this. Nice!