These things don't have "power supply" fans, they have a single system fan, located in the center-rear of the top of the case. They're standard box fans, (I'm pretty sure they're 12cm), so you can replace them with any similar fan of the same size and voltage. The only thing you'd really have to do is splice the new fan into the old fan's power cord, as the old fan had a fairly long cord going from its mount location to a jack on the power supply.
I personally don't mind the mild noise from a good, high-CFM fan like those found in the 630-form machines. Besides, most of the ones I've seen are really good ball-bearing Minebea/NMB or Nidec fans, so unless you desire complete silence in your work space, there's really no need to replace them - they last nearly forever.