i buy 99% of my cd's from half.com. if you stick to the "like new" or "very good" and read the comments, you'll get a rippable disc 99.5% of the time. over the last ~100 cd's i have purchased, i recently dropped to buying a couple rated "good" and one of them, the last two tracks won't play (visible scratches on the outermost part of the disc)
there is another site a friend of mine uses,
http://www.secondspin.com/
i haven't purchased from secondspin yet