I can't think of a processor fan that would be mounted on the underside of the motherboard. However, I have seen some fairly ludicrous methods for mounting a CPU Fan/Heatsink combo. Look at the pins that hold the fans frame down. There may be some difficult-to-see instructions on them, i.e. Arrows that indicate a necessary turning or perhaps lift pin that's just VERY snug and may take an ounce of gutsy muscle to disengage it.
I have one that requires you to turn the thumb piece to the left to unlock the lift pin, then you have to squeeze the thumb piece and the lift pin together with all the strength in your index finger you can find and that corner pops off the mobo all its own. All my smarts and took me a half hour to figure that out. Not to mention it was Best Guess which way I turned the thumb piece for lock and unlock.
As for where to buy. Try newegg.com Their Advanced Search function allows you to filter down to exactly what you need.
Good Luck!
