Yeah I was thinking about that, but I mean something that come with like the cooler master cosmos s, where that fan is mounted their in something. But I'm not getting the cosmos s.
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.
I've mounted a few on the bottom of the case...if there is good clearance underneath.
Usually just a hole saw, drill, standard 120mm fan, a few screws/rubber washers. But I've never really looked for fan designed specifically for the bottom of the case.
For what you want and what is available I haven't run across any box type fans seeing vents on the sides of the box you would mount at the bottom of the case. That type would raise the fan blades up away from being pressed right down close with a typica case fan.
Most pci slot fans aren't worth the match to toss them into a fireplace. If you are looking to cool hard drives or the bottom of the board more you may want to look for another case with better air flow.
What are you trying to see cooler by adding a fan at the bottom of the case? That would give a better idea for other options. hard drives, memory, video card?