Can you tell us why you need that one 10.3.5 update?
If you are trying to do the (dot) updates one at a time, (such as from 10.3.4 to 10.3.5 to 10.3.6) there's no reason to do that.
If you download and install the OS X 10.3.9 combined updater, you can update ANY 10.3 system directly to that last 10.3.9 version. http://support.apple.com/kb/DL496
That combined updater includes ALL updates from 10.3.1 to 10.3.9, rolled into one update.