Not available in bookstores anywhere
I've never heard of a published one. Every pharamceutical company that I've dealt with has their own standards for PLC validation.
Sometimes they lump them in with "computer validation" (which makes the standards difficult to use, due to the contant references to 'database' and 'audit trail'.
The good ones have separate standards for PLCs, SCADA/HMI, and "computer systems".
But most pharmas are also reluctant to just hand out their standards. I've usually had to sign non-disclosure agreements (once I even had to smuggle a copy out that a manager "didn't let me copy (wink, wink, nod, nod)".
If you find something, please post it. I'd love to be able to have something to reference.
Your best bet for SOMETHING is to buy GAMP (GAMP.ORG) and a copy of 21 CFR (Code of Federal Regulations) from the government. Pay particular attention to parts 210 & 211. Also Part 11 (the new hot topic). GAMP is not cheap. 21 CFR might be a Pueblo, Colorado production and available for a resonable fee (my former boss gave me my copy, so I don't know)
Good luck.