I certainly use those sites, and have found some rarities on ABE (although prices are usually on the high side). But a key to success in finding these books is to do what you're doing here - join online communities, let people know what you're looking for, and help out others to find their wants as well.
(Incidentally, if you had either a Drood Lettered or a Mister Slaughter Lettered, even PC, I'd throw all sort of money at you)
Terror S/Ls come up now and then. I've maybe only seen one other Lettered Terror on the market since I bought mine.
The Subpress editions of volumes 1 and 2 were published in May 2007 and January 2008, respectively.
HBJ