From what I saw online, these are a rarity now.
I found this statement in an old stratics post (2013) -
"They were the old Ilshenar maps. Before they allowed for treasure hunting in Ilshenar a few critters dropped maps there that didn't work. They stopped the map drop and those maps that dropped for the Ilshenar facet were considered rares.
Then we got a publish that allowed for tmapping on all facets and those pre-existing rare ilshenar maps were made into "artifact rarity 0" maps."
I would definitely ask a rares collector about them. The post in 2013 stated they were being sold for on average 25 million - don't know if this is still the case but definitely worth asking about.