I find it extremely embarrassing but two of my favourite sources of cheap books happen to be hypermarkets, to be specific, both Tesco and Giant in Puchong. Both of these hypermarkets happen to have their cheap books sourced by the same distributor, who I sometimes suspect to have shady connections to unscrupulous book warehousers, but that's just irresponsible speculation on my part.
In any case, I don't mind, like how I don't mind that Times and Popular and Big Bookstore keep having Book Warehouse Clearance Sales every other month at suspiciously cheap prices.
At RM8 or thereabouts, the books are cheap, but this is mostly because the books are horribly creased in the spine or there's a noticeable cutting or printing error. The books are still readable of course, it's just that they're not in pristine condition fit for the bookstore, and are, I suspect, rejects from the publisher that are supposed to have been destroyed.
Nevertheless, dig through the pile (and my! what a huge pile it can be at times), you might find a few gems, gems you might be hard-pressed to even find in Payless. Most recently, I found Flann O'Brien's At Swim-Two-Birds and a nice copy of Dashiell Hammett's Thin Man, as well as a couple of seldom seen Penguin modern classics (the titles escape me at this moment in time).
Sometimes I just go to Tesco or Giant to restock on coffee or earl grey, without the least intention of buying books, but then I step out with 5 books or so. Just goes to show... I can't escape book buying no matter where I go.