Southshore - Shady dealer - Upstairs at the inn
Mendithil - covered up there
Booty Bay - covered up there
They are the only 3 i know of, though there may be more
I have a 14 slot bag i fill up from time to time, with 2 stacks of blind powder, 2+ stacks of Cripling / Instant poison, 1 stack each of the other poisons, and a big pile of flash powder.
It cuts down travling time...