Someone asked about buying "sour powder" or dried sourdough starter.
Yes, I have bought Goldrush brand starter packets on a couple of
occasions. I've obtained them from an up-market kitchen goods mall store.
I also posted yesterday, to rec.food.sourdough, a fairly
long description of my experience inventing a sourdough recipe
for my machine. I'd send it to the list if there's interest, but
it's mostly just a narrative of my first effort.
I'm very interested in the factors that affect the bread, mine came out
too coarse in texture and didn't rise enough, but at least it was edible,
which is probably not bad for a first attempt experiment :-)