You should easily be able to get a generic replacement to your cooling fan or case fan from any reputable online computer store such as Dabs, Simply, Watford etc. Normally when you look for processor fans the specs will tell you for what type of processor it is for and the maximum speed of processor it can safely cool (faster processors generate more heat so need more efficient coolers). You could try booting your PC with the case fan disconnected (do this briefly and NEVER disconnect the processor fan). This should help to identify exactly which component is making the noise.
If you do need to change the processor fan, leave the heat sink in place if at all possible and just screw a new fan on top. If you have to remove the heat sink as well, you'll need to get some thermal paste.