Alpharam is correct. 40mm fans are the most popular fans in 1U enclosures. If you don't care about noise, a 40x40x28mm fan at 10,000 rpm has a max flow over 15CFM, max pressure over 0.5". Of course, you can also put multiple fans in series or parallel to boost performance.
There are also 25, 30, and 35mm fans but performance drops significantly.
There are many good sources. Try NMB, Delta, EBM, or Nidec.
Good luck
ko