You should be able to get one from a Toyota dealer, but this could be expensive. Try ringing around a few scrap yards to see if they have any good secondhand items. Remember when fitting the new item that they key to success is preparing the surface. In the meantime, you could try and track down the leaking area and use some silicone sealant to replace the leak. Remember to either leave the door open or use wax paper to avoid glueing the door shut.