By Although some shops do manage to sell blank cds or dvds at reasonable prices the best deals are still to be found online; especially if you're buying in bulk.
I tend to check about 5 or 6 different sites before purchasing blank media and two of the sites that I use more often are:
Both have a large selection of media available and tend to be cheaper than the others more often than not. They also sell a number of other PC and general entertainment related items.
I tend to use CD-Rmedia.co.uk