Amazon for genuine stuff. I've gone right off the copy stuff - worked well in an epsom, but the new HP we have, I stick to genuine - not too bad at about £10 per colour (3) and then it has a black and photo black (the photo black lasts ages as we don't print loads of photos off)
The stuff that comes with the printer is just to start you off. If you are a light user, you might be better off on the genuine stuff, or quality refils.