Are you sure the motherboard needs replacement? If so, then you are pretty much limited to the original motherboard, replacement one here...http://www.laptopteks.com/Gateway/Laptop/Gateway-MX6000-Series-Notebook-MX6960/Motherboard-(System-Board).asp
(I have never used this vendor)
A little more info here would be helpful, did your laptop die? Running super slow? Or are you wanting to upgrade without buying new?