AFAIK, from my limited experience modding PSUs years ago, you can use any size/pin-compatible fan you want.
I remember putting Panaflo 80mm units inside noisy Antec PSUs and they worked fine. You will obviously void the warranty, and you have to take extreme caution when working inside a PSU because components can still carry a charge even after the unit is powered off.
Open up the PSU and see what kind of header the fan has. Then go to Newegg and find a fan you want with in the same size with the same header or wiring setup. If the current fan's connector is soldered into the PSU's circuit board, it's very easy to splice fan wires from the new fan onto the old connector.