These top-tier priests are a valuable resource and not easily come by. In the Exiled Lands, Derketo high priests can only be found in the area around the Pagoda of Boundless Lusts, though they spawn only very rarely. Sometimes, they'll show up in a Purge, as well, but this is, if anything, an even rarer occurrence. On the Idle of Siptah, Derketo high priests rarely spawn in the camps in the southwestern quadrant of the island, and—again, even more rarely—show up in a Surge at a Leyshrine. In short, if you want to snag a top-tier Derketo priest, you're almost certainly going to need to be patient.