This is your processor: Intel® Core™ i7-3820 Processor (10M Cache, up to 3.80 GHz) Product Specifications
You will notice that it has socket 2011.
A quick search reveals these two fans, although there are a large number of fans that work for socket 2011.
Shockingly, these two fans appear to be identical:
Seems a bit pricey.
In this list of 23 fans that are socket 2011 compatible, the price varies widely:
Since Scott knows this motherboard and socket much better than I, we should wait for him to chime in. He is on travel and may take a bit to respond.
The links are provided only for your convenience.
A lot of these fans on the NewEgg page are designed for rack-mount 1U/2U workstation boards and will not work on the DX79SI board. Here is a sampling of some that I think should be ok: