Orange zest

OK, so I was going to brew up blacklab's orange/cascade pale ale. It looks so good, and im pretty excited. Ive already got all the ingredients lined out except for the orange zest. My LHBS was out, but i guess Id rather use a fresh orange anyways. Now, would zesting an orange from the grocery store would be sweet orange peel right, not bitter? And about how many oranges would it take to get 2oz worth of zest?
Thanks

personally i have a scale for hops so i would weight it out but it sounds like in your case you dont have any way to weight. If it was me, and i was without a scale, i would zest one orange for the boil and possibly add more later after primary conditioning was finished into the secondary. Fresh zest is different from dry because there is much more oil present which will lend its presence to the beer. Therfore it is my belief that 2 oz of dry zest may not be equal to 2 oz of fresh although someone please correct me if im mistaken.

If you zest only the outside of the orange you will be getting the sweet essential oils from the orange and none of the bitterness. If you zest down too far you will start getting into the white pith. That's where the bitterness comes from. You should be able to get 2 oz of zest out of 3-4 oranges (about baseball size). Adding it at the very end of the boil, just before you chill, will sanitize the zest and give you the most flavor & aroma. The longer it's in the boil the more aroma and flavor are boiled off.