Hi Piman,
I agree, this part is not easy to find. There is two ways of getting it though:
1) You can buy part 54118 (DB01 WO Assembly Universal Shaft - front). This part includes 9440664 (2 of them). The rear universal shaft includes the part as well I believe. This is a bit expensive though (about $30-$40).
2) Otherwise, you can do a special order from this website:
http://www.goldstarstockists.com/menu.htm. The website is not the best, and you won't find the part on it, but send them an e-mail. They are very nice and helpful and they will get the part for you. They will get in touch with Tamiya Japan directly.
Hope that helps,
Peter