
Lurk3r |

Kiss up to him. Offer to become his servant/ slave/ dishmaid. Pretend to be a scale wax salesman and give him a free demonstration. Offer to wash his feet or something else benign and humiliating, anything which will give you access to his lair. Dragons naturally shed scales every now and then- pick up the first one you find and make a run for it. You don't have to take it by force/ coercion, you just need access to his garbage bin.
The setup of 'can't fight it, can't bribe it' screams RP potential. Also a good chance for the Bard/ Rogue to get useful.

Drejk |

Have you actually tried to speak with the dragon? It might be Chatic Evil but it has certain interest in saving itself and it's sandbox (i.e. the world) so it might be willing to negotiate. Or you might want to wait for the dragon to shed it's skin and steal scales afterwards. And if it is a Red Dragon's Scale needed instead of the particular Red Dragon's Scale you could start looking for smaller dragon to kill/negotiate.

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Look for a measureing scale that a red dragon once had in his hoard.
Kidding.
Look for one in the market. Black markets and underground markets might have one. Look for people you can hire to hunt one down for you. Specialists in the wild and the bizarre should exist, and even if a dragon scale is rare, there should be someone who has one.
If it has to be from that specific dragon, reasoning wouldn't be too out of the question.