Hamilton is a clearance warehouse of books that are about to be pulped. They publish a tabloid catalog on newsprint that lists all the books at firesale/distressed prices. They have more than just history books, but it is a great place to find history books you didn't know you wanted. Addictive. Get on their mailing list.
Scholar's Bookshelf is another great source of the HIGHLY obscure. No discount prices here, but the titles available truly are aimed at the scholar trying to complete his library. Also many great and obscure documentaries, foreign and domestic. Another great mailing list you want to be on.