@feizalwu it appears as if iFixit does not sell parts for this device. Since the people on this forum are volunteers and not directly connected to ifixit, you could ask the iFixit sales staff. Use this http://help.ifixit.com/collection/130-contact-us to contact them.
There are no schematics for the A1121 and replacement power supplies are very difficult to find (if at all).
The power supply is a liteon PA-3150-01A 30W DC output so there may be a way of trying to wire in a different power supply. Check your wiring and let us know what you find.