Unfortunately most charity shops these days (or certainly the major ones) have somebody knowledgeable picking through stuff like vinyl records, CD's etc. Anything that appears to be worth a bit then gets listed on Ebay.
You might find something in a small, local, independent charity shop or occasionally stuff turns up at car boots. There's a lot of luck involved.
I believe there was a thread some time ago discussing pre-recorded MD's and which Atrac version was used. I've got a pre-recorded copy of Meat Loaf's Bat Out Of Hell album and I must admit the sound quality is nowhere near that of the CD.
I haven't actually tried this but I reckon if I copied the CD onto a blank MD using my deck (in SP mode) the copy would sound better than the pre-recorded one.