Along with your usual HMV and Virgin (both of which are geting worse) I can always be tempted to lighten my wallet in Fopp. Good secondhand shops in Sheffield are few and far between these days, gone are the likes of Hitsville and Amazing Records where you could get cheap affordable stuff along with the collectables. Jacks is overpriced (and way too small since the move) and the only other place I know these days is Record Collector, what else is there? Manchester has loads of seconhand shops, but most are overpriced.
On the net I find CD Wow best for new stuff, Plastic Bird is good for second hand indie type stuff, and there's always GEMM, Netsounds and Musicstack - if you can't find what you want on there it probably doesn't exist!
I tried CD101 a few times but they never managed to produce the goods. I eventually spoke to someone there who admitted that what they list online is just a catalogue, not actual stock, so if it's old or rare then they probably won't ever get it.
You can't beat a good car boot sale though!!!