If you take out the old fan and:
Copy the data on the sticker on the Hub (manufacturer and part number).
Measure the distance between adjacent mounting holes in mm (e.g., 90mm)
Measure the thickness in mm (e,g., 25 mm)
You can look up the part number and find the maximum air moved in Cubic Feet per Minute (cfm), and the maximum sound level in deciBells (dB). You probably can only order an exact replacement from the manufacturer in the "handy" 10,000-pack.
You will need a same-physical-sized fan with at least the same air-moving ability, and quieter is better. A commercial office has about a 35 dB sound level, a chain saw about 95 dB, and the scale is logarithmic. \[Slightly bigger numbers are WAY louder.]
This site has an assortment that may help you see what is available:
http://www.buyextras.com/dc12vfans.html