Stumbled on this while searching for other things...
The William Vareika Fine Arts, Ltd Gallery
I've actually been to this gallery in Newport, Rhode Island (while LaFarge hopping) and so I've seen the many, many LaFarge sketches for stained glass they have at the gallery. There is a web gallery of LaFarge sketches, though the images on the website are kinda tiny. Still, you get some sense of the design approach. Worth a look.