SEMI standards
The official source of the latest SEMI standards, including F47 and F42, the voltage sag standards. (For a free, fully-licensed copy of SEMI F47 and F42, send an e-mail with your mailing address, etc. to FreeCD@PowerStandards.com.)
Info about SEMI standards -- generally, you'll want "Standards Publications / SEMI International Standards"
SEMI's Web Store -- purchase and download SEMI standards (not always quick)