Hi luca, sorry for my english, it's very bad
if I understand the rubber membrane you need correctly (a bit of rubber, costs about 70 eurocents), I think you cannot find as spare part and buy in scooter spare shop. Try to contact directly Grimeca helpdesk on their website, and ask if they can send you the spare with post service. www.grimeca.com
normally they'll send you a PDF file with parts schematics and their codes, so you can identify the parts you need and they'll ship after paying (I don't know how you can pay them).
my regards and welcome to VR.
