Where is the official RS-232 specification?
The original standard has been written by the Telecommunication Industry Association (TIA). Its name is EIA/TIA-232.

But you must pay to get it. You can buy it on TIA's store. This is the reason why you don't find it on the internet: publishing it is illegal.

Anyway, all the necessary information required to implement an RS-232 device is easily available on the internet and on various 3rd party publications, as you noticed. I guess TIA doesn't make much money from this standard anymore.

(and at this price, I wouldn't buy it)

Here are some useful resources:


Linear Tech RS232 Quick Guide, summing up pretty much everything needed from a technical point of view, in a single page.
Wikipedia page for more detailed explanations and some historical background.
An old app note from Dallas (now Maxim) covering all technical aspects in details.