Darrin started handling this hard-to-get stuff (and, importantly, Seitz windows, too) as an adjunct to his own camper builds, but he has helped out several people and--unfortunately, given that he's not really in the business--he's about as knowledgeable as anyone in the country.
So he's worth contacting, but remember that he's doing this as a favor to the community. He doesn't make any money off it, it's not a mainstream activity for him (he has a 9000 acre cattle ranch), and he's not responsible for the fact that the distribution and shipping of these products is often screwed up.
The point being . . . he's a good source, often the best source, and he's worth contacting. But be polite, tolerant of the apparent inevitable delays and mistakes at the distributors' end, tolerant of the fact that much of the stuff has arrived damaged from poor packaging, and mindful of the fact that Darrin does this mostly as a favor to overlanders.
In fact, the hassles with the windows have been so great recently that he told me he's near giving up on them. Which is exactly what we don't need. So use the Darrin resource, but with appreciation and realistic (which is to say lowered) expectations of how well it will go.