Google shows 1400 hits on pandemonia but they seem to have other excuses for being* than being the plural of pandemonium. Pandemonium is one of those uncountable thangs. Either it's there or it ain't. It's not even like you'd say there was a pandemonium in the marketplace yesterday. With the one we had in the Forum last week, that's two pandemonia this month alone.

*One hit was a reference to Prehysterical Pogo (in Pandemonia) and there's apparently a computer game Pandemonia. There was also a reference to what may be a goddess named Pandemonia, but that would be feminine singular, not neuter plural.