If you're not in a tearing hurry, 7dayshop have decent prices (but do add P&P). I tend to keep a complete set of spares in stock and reorder when I need enough to make it worth an order. (CLI8 - 6.99/cart; BCI6 5.69/cart [depending on colour])

I tried a cheap replacement once and didn't like it at all. Poor colour rendition made me take it out and flush the head through by printing a sheet of fading cyan to get as much as possible out as fast as possible (in case of head blocking).