Before airlines put into service their A320s they consult with Airbus who either invite over or send a rep that sits down with the big chief to produce a tailored version of documentation derived from the source documents for a particular fleet. As for SOP, I don’t think the FCOM is something the airline can re-write. For example, you will not find a custom SOP described in the Airbus Normal Procedures section of the FCOM – not for any airline. For this, Airbus will work with the airline to produce a customised FCTM or other documentation. Most don't bother. They simply rely on teaching airline SOP passing it off as Airbus SOP.
So in answer to your question, where can I find an Airbus SOP? In the NP section of the FCOM!
A question for you is how much has to change before you no longer consider your procedures to be Airbus SOP? If an airline chooses to add a couple of additional checklist items, are they no longer following Airbus SOP? Where do we draw the line?
An example of a big deviation from Airbus A3xx SOP is airlines that do not allow the FO to start engines/taxi the aircraft (for whatever reason) when the books clearly show PF/PNF actions regardless of phase of flight. Again, where do we draw the line?