To get Ghost Flowers, you will need to be playing in New Game Plus, which means you will have to have finished the main story at least once. Once that is done, you will be able to start an NG+ run from the main menu. NG+ mode is pretty traditional in that you will keep your equipment and unlocks, but you will also get access to NG+ exclusive features, such as a new vendor who takes Ghost Flowers as currency. You can find the Merchant just to the northwest of your home in the Yotei Grasslands. To get Ghost Flowers, you will need to complete main quests and side quests in the story, so get stuck into the exciting narrative of Ghost of Yotei. Doing bounties for Kojiro the Bounty Broker will also earn you Ghost Flowers, as will unlocking Altars of Reflection. You will get some for each Altar you unlock, so don’t forget to visit them all again on your new run. The vendor, Masujio the Melodious, will sell charms that can also grant these flowers. They will usually come with some sort of curse as a tradeoff, such as an increase in Stamina costs, and result in a chance to get flowers from killing enemies. These are the Charm of Curse Spirit, Charm of Misfortune, and Charm of Frailty. They essentially allow you to trade less value drops in NG+ for the chance to get Ghost Flowers.
https://www.shacknews.com/article/146963/how-to-get-ghost-flowers-ghost-of-yotei
