during a bit of a clear-out at home my extention tubes disappeared, so I had to buy new ones, as I only use them ocassionally I didn't want to pay a great deal, so I went to Linkdelight. This is a Chinese company
http://www.linkdelight.com/L9B-AF-Ma...OS-Camera.html delivery takes about a week. Very good quality at a very reasonble price of £38-61, not sure how well they will work with your zoom lenses (I use mine with my 100mm macro lens.
As an alternative have a look at the Ranox. It gets excellent reviews, samples here
http://www.cameratips.com/recommenda...raynox-dcr-250 cost approx £42.
Harry