I got mine from my flying school whilst I was still doing my private pilots licence.
I get all of mine from pilot supply shops now. Transair is one of the bigger suppliers of pilot equipment in the UK they have shops at a couple of airfields (Fairoaks and Shoreham definatly) I think they have a few others as well.
They usually cost about £15 for the standard half mil or quarter mil VFR charts. Don't know about the IFR charts, I think they might be slightly more expensive (I don't use them because I'm not instrument rated.....YET).
Hope this helps
