Don't know where to get the sticker. But if Fibro is not considered a disability, it seems like a risky thing to depend on such a thing as a sticker in order to park in the handicapped spot. In the US or Canada, if you are not registered with a disability and do not have the correct plates or windshield hanger, it is illegal to park in a handicapped spot, and you can get a hefty fine for that. A sticker like what you saw wouldn't help. I don't know what the laws are in the UK, but I would not be surprised if they were the same.
If you are physically able to walk from a regular parking space, then do. The exercise is good for you. If you are literally not physically able to do that, then maybe you could try again to get registered as disabled, and prove that you cannot walk. Maybe without even mentioning Fibro, since that seems to get in the way where you are.