The barrel nut (insert in the fin) is machined with a larger tolerance in the threads than a "normal" nut so it will easily strip if you hit something, thus saving your board..
That would be my guess as to why a specialist bolt retailer reckons they don't have something that will "fit" - cos it is kinda designed to not fit properly if that makes sense. They would have measured it and thought it non standard (which it is really!) Get a fin barrel nut and a standard M6 nut and check out the wobble on the former!
Your chance of getting M6 at larger hardware shops like Bunnings etc is just about nil as they do patio bolts etc, and if they do have it the staff would not know a bolt from a nut anyway

Buy standard M6 - which is 6mm metric and standard pitch of 1mm (25.4tpi).
Any marine shop or proper fixings place will have bucketloads of them.
If it does not want to screw in, as others have said just rotate the barrel nut with a screwdriver, that's what the slot on the side of it is for