The only official methods involve increasing your proficiency bonus or your Intelligence score.
You have already observed that the Ioun Stone of Mastery increases your proficiency bonus, which in turn increase your Discipline Save DC. The only other official method will be to increase your Intelligence score by one of the various methods. The most reliable method is to increase your Intelligence to 20 via Ability Score Improvements, or if your Intelligence is below 18, a Headband of Intellect will raise it to 19 for a +4 Int modifier. There is also an Ioun Stone specifically for Intelligence adding +2 to a maximum of 20. Additionally, the Tome of Clear Thought can increase your Intelligence beyond 20, adding +2 each time it is studied.
Consider asking your DM to adapt the Dragonhide Belt to the Mystic class. The Dragonhide Belt was added in Fizban's Treasury of Dragons specifically to give monks an item that increases their Ki save DC:
This finely detailed belt is made of dragonhide. While wearing it, you gain a bonus to the saving throw DCs of your ki features. The bonus is determined by the belt’s rarity.