Try Case-mod.com. They list by chipset type.
Still, you might have to do a mobo removal to
install the fan if the stock clips are not compatible.
Personally, I've replaced that screaming loud
fan you have with a newer model ( heatsink
and all ) and it was much quieter. I think the newer
stock models have different bearings and wider
blades.
Case-mod has a lot of stuff but I didn't see anything for Slot A. Could
you post a link?
In addition to the 50mm fan on the CPU heatsink there is a 40mm AAVID
fan on the GPU heatsink which may or may not be a contributing factor
since I have not listened to it by itself. The heatsink on the GPU is
big enough to accomodate a larger fan - probably at least 60mm.