The Source of Magic is the 17th chapter of Tsubasa: Reservoir Chronicle.
The warriors all go in to attack Syaoran at once, but Syaoran counterattacks, positioning his body so that he can attack all the warriors in a row, one right after another. The son of the Ryanban refuses to admit defeat, and calls upon the power of the wind to hit Syaoran. Syaoran falls, bleeding, as Sakura goes to see if he's alright. Chu'nyan yells at the son, saying that the only way he can win is by using his father's power (ie. the wind). He ignores her and tells the vendor and grandfather that he's increasing their taxes yet again and if they don't come up with payment soon, he will whip them 300 times.
Dejected, they return to Chu'nyan's house where Fai and Kurogane are awaiting them. Fai notices that something must have happened, as they recount the events in the town. Ever since the ruling Ryanban has arrived, their town has suffered. Fai observes that is it too much of a coincidence of the timing between the Ryanban's sudden emergence into power and Sakura's missing feather. Confused, Kurogane asks him how it could be possible for the Ryanban to have had power that long when Sakura's feathers were scattered only recently. Fai suggests that different dimensions have different streams of time and it is possible that one day to them could've been a year for Chunyan's dimension.
To defeat the oppressive Ryanban and to take back Sakura's feather, it appears that their only alternative is to go and attack the castle. Syaoran gets ready, but Sakura grips his hand to tell him that he's hurt and shouldn't go to battle. He reassures her that he'll be fine.
Before they go off to storm the castle, Fai reminds them that they still need to remove the magic barrier that protects it. Since he doesn't have magic, he suggests that they contact their good friend, the witch of dimension, for some help. Through Mokona , they are able to communicate with Yuuko. She's surprised that Fai's consulting her since he has his own magic that could do the task. Fai reminds her that without his tattoo, he decided not to use magic anymore (~henya!). She then gives them magic, but demanding yet another payment in the form of Fai's staff. Fai sends it to her dimension by feeding it through Mokona's mouth, much to the horror of Sakura and Chu'nyan.
Yuuko then sends a ball of magic back through Mokona, which should be able to help them get into the castle of the Ryanban.
Directly crosses over with Chapter 15.