Go to Newegg.com.
From Computer Hardware Menu, select Motherboards
Under Shop Category, select either AMD motherboards or Intel Motherboards.
Under Narrow Results, select the Advanced tab.
At the bottom of the Narrow Results list, select Mini ITX from the Form Factor drop down.
Look through the list of motherboards and their options. Obviously, you're looking for a board that has a PCI-Ex16 2.0 slot, so you can narrow your results even further.