To answer your question…

Someone emailed me asking if they make case fans that can put on the bottom? Unfortunately one site that carries specialty fans is down for maintainance at the moment. Case-Mod.com – Case Mods, Cases, Power Supplies, and Cooling for Home Theater and Gaming Systems

Antec has a rather different approach with one of their specialty fans simply being secured to one of the lower board mounts while being directional seen at Antec

With most pc fans found they are generally mounted where there is a preformed opening on the front, rear, side cover, or top of any case. But that would be simply custom mounting a typical case fan without any opening or vents seen below it.