Unfortunately, the Dragon Event ran from December 23 to January 5, so neither it nor the items from the subsequent event are available right now.
Right now the game does not have repetitive events, but instead implements a new event every month, in which you will have to participate to receive or experience them, although everything can change like in Honkai Impact, another miHoYo game, where there is a store from which you can buy specific items from past events. But this is only certain when it happens, if it happens.
That's all you need to know about how to get the Festering Desire in Genshin Impact, so we hope we have been able to help you as much as possible so that you understand how this type of reward works.