Hi Jane,
You can get them from the Repsol garage but you need to sign all sorts of disclaimers, I got mine from the little shack next door to the old Barclays bank in Mazzaron Port. Cant remember how much it cost but once you have then it is fairly cheap to get them to refill as they just swap out. I have also heard that you can obtain from some of the local markets but then you are still faced with the challenge of re filling.
Good luck Tony