You can buy a variety of mini display ports at your local computer electronics store. I bought a Belkin mini Display to HDMI, and a DVI no name brand that both work fine.

The adapters range in price and quality so going local the best bet in my opinion. This way you can swap the dongle out if need be.  Don't get me wrong I buy online all the time however I cringe when I buy a 25 dollar item online and pay 8 dollars in return shipping to swap it for another item.  

How I set my rig up: I achieved 2 external  monitors by plugging one Mini Display into the Dock, and the other into the Surface above the USB device.

Note: Some adapters will work on the dock and the surface, and others will work exclusively on the surface. Make sure you do some testing before the return policy is up.