You may need to know such things as:
Maximum RPMs (so can provide right cooling level)
Maximum current or power drain (so don't overload mobo circuits)
Fan dimensions (W, L, and H) Usu. something like 70mm x 70mm x 20mm)
Check out mobo (or computer, as appropriate) website for replacement parts
Call your mobo (or computer, as appropriate) manufacturer
Search online for your mobo and replacement fans
Search online for fan part number (if available)
Look through vendor websites like newegg.com
, etc. --- lots of them around
You may or may not be able to find a replacement fan without having to buy a heatsink as well. Hopefully you can find a replacement fan that will attach to your existing chipset heat-sink or chipset fan bracket.
Make sure connector fits your existing connector in size and number of pins. You may or may not be able to verify wiring polarity, and may have to re-wire the connector. That doesn't happen too often but I've run across a few non-standard fan connectors.
Don't buy a "sleeve bearing" fan under any circumstances