I personally think the best option to match the original image style, would be to take public domain pics (from the US military for example) and run them through a pencil sketch conversion. There are some online sites that do this or it can be done in photoshop with
many (6-20 depending on the image) simple steps. The steps can be scripted though.
http://www.picturetopeople.org/photo...to_effect.html
http://photoshopcafe.com/sketch-phot...oshop-tutorial
Converting to a sketch also moves it closer to being classified as a derivative work which might offer some legal protection if the images are not fully public domain.