I purchased these fans on ebay, although I am sure others will work. Use a 4 1/2" hole saw to make a hole large enough for the 120 mm fans. Templates are available online to make it easier.
Can you post what replacement fans you used, the three on mine now are very load i want to replace them but not use what with .
ty man i used one of them and a 140mm fan and i have two pwm switches coming from china , the fan i linked was to loud and i had two quieter ones laying around one was what you linked .