Source: Fizban's Treasury of Dragons

Wondrous Item, uncommon (+1), rare (+2), or very rare (+3) (Requires Attunement by a Monk)

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. In addition, you can use an action to regain ki points equal to a roll of your Martial Arts die. You can’t use this action again until the next dawn.