For an exact replacement, Dan is right - the factory will fix you up.
For reference for you and others if you consider changing the shades, they are fairly standard mounts. We ditched our dark brown shades (just like yours) in our Big Country for lighter shades that we found at Lowe's Home Improvement store. If anyone goes that route, just unscrew the nut and take the shade with you for a reference on the size of the hole in the top.