[Endoperez]

As easy as Pythium? None. However, all Arcoscephalian mages have Astral, R'lyeh has Star Children (the assasins), anyone who has found independent Shamans has access to 110 gp communion slaves, arguably Marignon (F2S1 Witch Hunters, but with S2 being the highest they are vulnerable to Magic Duel). Abysia has 150gp Warlock Apprentices, but you usually don't want to recruit one over Warlocks, both being limited to the same province.

I'm not sure if Tien Chi Masters of the Way have S1?1 H2 or W1?1 H2, but if they have Astral, that's one.
Iron Woods Jotunheim has Vaetti Hags. They have sorcery random, so one of S, D, N or B. The S ones are good slaves, B ones can hunt for virgins, N ones summon Vinemen, D ones can Reanimate and lead those undead giants. They are very cheap, so even with only 1/4 being communion slave, you probably have enough for communion.