I would really like to know where I can buy new silent cooling fans to replace the lawn mower fans I have running in my system. Some one here asking for help said, "I cant concentrate..." I can relate.
you can pretty much buy quiet cooling fans anywhere case fans are sold. try to find ones around 20db or less. endpcnoise.com has a good selection.
you could also consider adding a fan controller to use with your existing fans to quiet them down (or use with quiet fans to make them almost silent ). i have this one and it works quite well.
running the fans slower will reduce the overall airflow, but that's not usually a big deal. personally i'll take the reduced airflow (and slightly increased temps) in exchange for the silence