I am looking for a 802.11n simultaneous dualband wireless router with gigabit Ethernet ports, but is one that comes with open source firmware, OR can accept custom firmware that are open source. Do they exist? If so how/where can I find them? Once I do, are there guides that teach you how to set up the custom firmware? Thank you.
It depends on the firmware you have in mind. Most 3rd party firmware will have a list of compatible devices, ie: http://www.dd-wrt.com/wiki/index.php/Supported_Devices .