You don't have to find an SP3 disk
Most importantly...system manufacturer and model? If OEM (HP, Dell, etc.), then you need to check for instructions at respective website...pertaining to installing SP2 and/or SP3. Particularly so with HP systems.
Whatever SP is currently reflected on the system...you can download/install either SP2 or SP3 from your system.SP2 DownloadSP3 Download
If your system currently reflects SP1 installed, all you need is the SP3 download. If the system either has no SP installed, you need 2 first, then 3.