There are a number of sites I use...
mymemory.co.uk - Good for CD-R's and DVD-R's
amazon.co.uk - Good for DVD DL, probably cheapest I've seen
Don't go wasting the DL ones unless you have to, or you can afford it... they're a complete rip off, and its a bit of a waste to burn 4Gb onto a DL disc when you could just put it onto a DVD-R...
t.