Extract your previous service pack with a zip utility, whether it be sp2 or sp3. From there you'll have the msi file.
When you ran the service packs you installed from the one exe that you downloaded. That extracts the files to a temp and they are no longer in that location after the installation cleaned them up.
The important thing is that you point to the previous version.
As an FYI, when you install the service packs, save them on CD or something (just in case a service pack is pulled, or you can't find it when you go to install the next rollup or HF).