When you are puchasing an mandrel and hole saw....
1. Do not settle for cheap ****. The last thing you want is the blade shattering when you are cutting.
2. Tell the sale guy what you are cutting into. Again, you do not want it shattering.
3. If you have a DeWalt Shop right next store to a computer store (Like my DeWalt Shop), then the sales guy will know exactly what you want and will very likely have the stuff in stock.
4. Make sure you have power drill that is strong enough to cut through metal with the hole saw.
5. Safely Gogles and Ear Plugs are a must.
Check out DeWalts
You will be needing a Hole Saw Mandrel
You will be needing a Hole Saws...
3-1/8 (76mm) Bi-Metal Hole Saw for 80mm Fan
3-1/2 (89mm) Bi-Metal Hole Saw for 90mm Fan
4-1/2 (114mm) Bi-Metal Hole Saw for 120mm Fans.
I purchased mine directly from DeWalt. The Mandrel was AU$40, and the each hole saw averages at AU$40.
I was lucky as this was a Chirstmas Present from my dad