In the course of my reading for my drug design class, I came across a drug, etorphine, listed as ~1000 times more potent than morphine. Now, it doesn't look particularly simple to make so it's probably an impractical target for the underground chemist, but I was reminded of the thread a few months ago about just what drugs could be found on blotter paper. I have no idea what a typical dose of morphine is, but I imagine that a thousandth of it is pretty small. Assuming that etorphine is orally or sublingually active, and a good substitute for popular opiates, this would seem to be a particularly convenient analog.
If anyone is interested, the text references K.W. Bentley and D.G. Hardy, J. Am. Chem. Soc. _89_, 3267 (1967) and a second article appearing on p 3273 of the same. My Merck isn't handy but the drug is used to immobilize large animals, so I imagine good production techniques could be found in the literature or patents.