The serial number for most Cooler Master products will be found on a bar code like the example below:

Case Fans and Other Accessories
Case fans and separately sold accessories will not have serial number so they will only have limited store warranty or are replacement only if DOA
Here are same other examples below:

Always located on the back of the case usually near the back I/O or beside the PCI expansion slots

Located on the fan wire of the case fan that comes with the heat sink

Located on the wire of the pump/CPU block

Located on the backside of the keyboard

Located on the backside of the mice

Located on the inner headband of the headset or located on the backside of the volume control bar

Located on the side or backside of the power supply

Located on the backside corner of the notepal

Located on the front or backside of the adapter

Located on the warranty card that is bundled with each mobile power device