I've never seen one. Possibly because UL limits wall warts and dekstop power supplies to 660 watts and you can achieve that without a fan.
Isolation voltage and creepage and clearance distances can be all over the place depending on the safety requirements they are designed to meet. You'd have to have a data sheet to answer that question. UL 1310 and 840 have tables of distances. They vary with voltage, pollution classification, safety category, and things like whether or not the enclosure has any openings. That said, most of these distances are on the order of a few millimeters.
And, of course, there are plenty of power supplies that aren't actually designed to meet any particular standard.