Update: So far the winner is the PI7C9X3G1224GP from Diodes INC. It's a 12 port 24 lane PCIe gen3 switch, so slower than I would ideally like, but is in stock at digikey in qty 1 for $100 and also has a 324 page datasheet (i.e. probably reasonably thorough) available without any NDAs or restrictions.