I'm not sure how Sean comes up with three dozen DD dealers in the USA. I count 151 on Detroit's website listed as providing service for DD engines.
Trnasynd is not the only ATF that meets Allison specs for extended drain intervals. There are four other fluids that Allison has tested and meet their TES-295 specification. Now, I have never been able to find anyone who carries any of the four.
When I opened my guide (.pdf format), I somehow counted only one page (of three).
There are, actually, 107 (if I counted right this time) DD distributors in the lower 48. I stand corrected, but there's still never one near where I need it to be. For example, I could use one right now in Santa Fe, but the nearest is in Albuquerque. And there is not a single one within 30 miles of my planned route from here to Nashville. By contrast, I pass NAPA stores all the time, and about half of them (in my experience) have Rotella-T 40 in stock. YMMV.
TranSynd is absolutely the only fluid that meets the Allison spec for extended drain. Allison destroyed the test stand after certifying the Castrol product, to the ire of many competitors. This fiasco has been well documented, and debated to death. The four "other" fluids listed as meeting the spec are actually Castrol TranSynd re-branded through cooperative marketing agreements. Castrol is the only manufacturer. (That being said, the mere fact that it is available through these other channels does put competitive pressure on the price.) FWIW.