I think you're correct, but I think it'll take EDMC et all auto harvesting the data from passersby before we'll be able to spot any pattern, partly for the sheer numbers but also to eliminate people purchasing them and distorting the values, if it is indeed a limited purchase opportunity.The two shops I've found have both been in Democracies; one in none state, one in Boom+Civil Liberty.
In fact there's no evidence of a pattern so far. Government, station economy, and states aren't conclusive factors; local rep doesn't seem to matter at all.
There may be one or more factors that increase the likelihood of upgraded items appearing in a way that would be hard to test. You'd need massive quantities of data.
This is interesting! It suggests that items are persistent system-wide until they're purchased by someone.
So far, I'm not finding anything but Dominators for suits at these Corporate/Industrial stations, could be coincidence again, so I may swap to some of the types you've visited to see if it finds others.
Regarding duplicated inventory, who knows with how wonky this release has been! But always best to report all data rather than prejudice it!