It was my impression that the surface of the glass plate was first coated with optical-quality pitch, into which the starch particles would be imbedded. I had also read that optical-quality beeswax was added to the pitch, in order to improve adhesion.
Pitch is used by amateur telescope makers. A good source would be Willmann-Bell. They have both the pitch and the beeswax, in addition to a mixed version, called Burgundy pitch.
Individuals may have difficulty purchasing from Sigma, due to the high level of paranoia in American food-related industries. I purchased some potato starch from them several years ago, and had to justify my purchase to a high standard. Fortunately, at the time, I was in a food manufacturing industry (I owned a brewery), and they allowed the sale. Perhaps they've loosened their standards by now, but that's to be determined.
ps: I have the Lavédrine book, and it is spectacularly beautiful. Alas, my French is almost nonexistant now, and I struggle to get through a few pages now and again. On which page is the "pale latex crepe" reference?