Finally found them: 28 to 35 mm Challenge latex tubes. These tubes are difficult to find except in late summer and early fall, when retailers start stocking them for cross.
I like latex tubes, and yes, I can tell the difference. So much so that, on my English, I use latex tubes in my carbon fiber clinchers–a practice generally considered inadvisable because of the potential for melting. However, I don’t brake much, and I’ve been pairing latex tubes and carbon fiber clincher rims for years without incident.
The other nice thing about latex tubes is that–at least in my hands–they are resistant to puncture. The tube that currently resides on the front wheel of my English has survived three years. No punctures, and most of that time was riding in New York City. Contrast that with the butyl tubes on the Herse (no choice because it is 650B), which puncture at least once or twice every thousand miles.
There is considerable disagreement regarding whether latex tubes reduce or increase rolling resistance. Jan Heine’s real world tests indicate that they increase rolling resistance slightly, while other more carefully controlled but less realistic tests suggest that rolling resistance is reduced. The latter result makes more sense to me in the context that more supple tires generally have lower rolling resistance, but who knows?
Online, you will see complaints about punctures occurring when mounting latex tubes. There are a few ways to avoid this: First, try to mount the tire by hand. If you need help, a tire jack is a safer bet than a lever. Second, after mounting the tire inflate it slowly to 20 psi, let it rest for a few minutes to release any tube trapped under the bead, then add another 20 psi and let it sit for a while longer. Grab the tire with both hands and wiggle it to ensure both beads are seated. Then pump to full pressure. Thereafter, you can inflate as you normally would. I’ve never punctured a latex tube using this method.
I received the Challenge tubes today, and on a short break from work mounted them under the Grand Bois Extra Legere tires I received recently. I have only ~100 on the tires so far, so don’t ask about them yet. I’m planning to take Friday off for an 80-mile double crossing of the Shawangunk Ridge. I might have an opinion after that ride.
John
Riding the Catskills 
I think a resolution of the rolling resistance question may be it depends on the tube. In general lighter tubes have lower rolling resistance. If Jan is testing particularly beefy latex tubes, while the BikeTechReview tests are on thin Michelin tubes, perhaps that explains it. I agree with you it makes sense latex would have lower resistance, and this is observed not only with clinchers, but also with tubular tires (which can have latex or butyl inner tubes) over and over in the BikeTechReview tests.