Try www.spacestaion42.com ,and click on the free paper models bar at the top of left margin. that will take you to the fre paper models page where you can then select from
different categories.
Also try 3D paper model net.. There are literally a few hundred paper models you can download,a good number of them being sci -fi subjects.
There is a Shuttle Tyderium on there that will absolutley blow your mind.. BELEIVE ME..It is the paper model EQUIVALENT of a filming prop miniature..Sriously.