I found this forum when looking for exactly the same thing myself. I have a Sigg Metro Mug, which at about 8 ounces (.25 liters) seems to be the smallest mug per se. Very nice. Still, it's too big.
I looked around in laboratory equipment. They have what's called a Dewar flask, which is basically the ancestor to the Thermos. The bad news? $123 and up.
So now I'm thinking jerry-rig a test tube. You can get test-tubes in all kinds of sizes, with diffferent kinds of stoppers, and you'd only have to insulate them. A neoprene or silicone sleeve? High-density foam?
Saw this old post and thought I'd add a bit too it. Here in Australia there's some specialty coffee shops that sell Espresso travel mugs, I there's some places online that sell them as well from time to time so you might want to try that as well. But I don't know where you're from, so not sure if this info helps.
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