You can download the RC (release candidate) version of SP1 here:
Windows Vista Service Pack Download - Softpedia
You will however get a small text appearing on your desktop saying 'Evaluation copy'. If you go to this web page it is explained, along with a small download how to remove the 'watermark':
Patched user32.dll.mui to Remove Evaluation Copy Text on Desktop of Vista SP1 RC1 » My Digital Life

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