You don't need the specific disc that came with your server per se. Instead as long as you have a disc that has the type of OS your server has on it you should be able to use it to load the required features.
By type I mean the license type: OEM, VLA, etc. Although the type may not actually matter for installing IIS.
There may be alternative means to acquire a 2003 x64 ISO but I would not recommend them here as SysAdmins should only be using discs that they can ensure are clean and from a vendor or reliable source. I also don't think there are any work arounds to install IIS with any files on the HDD already. Prior to 2008 features had to be loaded from the disc.
If you have TechNet or MSDN access you should be able to go download
an ISO for what you are looking for.