It depends on the way the heatsink and fan are mounted. If you just want to repace the fan, a small fan can replace it, depending on how much you want to "mod" it. Or you can get a heatsink and fan like these.
http://www.newegg.com/Product/Produ...ice=&MaxPrice=&SubCategory=62&Submit=Property
Look for low decibel levels, I usually try to get <25db, but you may want lower if you want it silent.
http://www.newegg.com/Product/Product.asp?Item=N82E16835110106
http://www.newegg.com/Product/Product.asp?Item=N82E16835186117
The second one has a review of a guy that used it on a ti 4200.