Not only should you not need XP service pack 1 in the normal course of things, you shouldn't be asked to insert it. This sounds as if you are using some manufacturer-supplied CDs to restore the computer to its "fresh from the factory" condition.
What is the state of your computer at this point? That is, when the process stops because you don't have the next CD, is the system stable running XP with no service packs?
FYI, XP service pack 1a (which is no longer available, AFAIK, from an "official" Microsoft source) is identical to XP service pack 1 but for the removal of the "Microsoft Java Virtual Machine," which was Microsoft's version of Java. Shortly after the release of sp1, Microsoft settled a patent infringement suit with Sun (the creator of Java) and part of the settlement required removal of the MSJVM from sp1, hence sp1a. Thus, if you are getting some message about a missing file if you use an SP1a CD, something else is going on, e.g., the "Service Pack 1" CD that is being requested is not a Microsoft product but something put together by your computer manufacturer.
Microsoft no longer provides XP sp1 for download because XP Gold (no service packs) can be directly updated to service pack 2. The procedure for getting XP Gold fully updated can be found here (but this is a very old post, so some of the links no longer work) --> https://groups.google.com/forum/#!msg/microsoft.public.windowsupdate/4-jCtXsbshs/HBI-0476Wj8J
BTW, there is no such thing as "SP1 for Windows XP Office Edition," so don't try to use that.
It's possible that the file named xpsp1_en_x86.exe here is a legitimate version of XP sp1, but I have no information whatsoever about the reliability of this site or what the file actually is. For all I know, it could be infested with malware. Moreover, I really doubt that it will help you, even if the file really is a good copy of the official XP sp1 file, because your description of your problem sounds as if you need something else. --> http://slug.ceca.utc.edu/ftp/pub/windows/patches/winxp/