So I need a dozen of them. Is there a more effective way than stealing journals of infamy and converting them?
Converting stolen Journals of Infamy into Correspondence Plaques is probably the most effective way of getting them quickly, yeah. Funnily enough you could also convert the Journals directly upwards for a rare shot at Storm-Threnodies, but that would probably take a while unless you’re lucky.
I think it’s slightly faster to upconvert from maniac’s prayers, if you get those by repeatedly breeding somnolent hyaenas. This may not be true if you only need 12, although correspondence plaques (and aeolian screams) are useful for other things.