Most of my collection i've bought in bulk from friends emigrating or folk selling their Hi-Fi kit. I used to get a few bits from CD Wow but tbh fround their website hard to use and was annoyed at endless CD's turning up in broken cases.
Last few CD's i've bought have been from the Lamb's CD store in York. Specialise in Classics and Jazz, great little independant shop. Well worth calling in if you're nearby.