the spanish diet and cuisine is based around high fibre foods. fresh fruit, veg, pulses by the score. for a fascinating insight into this and local social life at the same time hit ibiza's new market in the centre of town. there you'll find fish. meat (i reckon spanish meat is the best in europe), oils from all over spain (olive oil like you've never had in your life), herbs spices, breads (with lots of grain), a mountain of fresh, locally grown fruit and veg, cakes and pretty well everything else you could think of. allow plenty of time for shopping here.
for a more familiar shopping experience, go to the hiper centro on the main road to san antonio, (quite near to pdv), you might even find a hershey bar there.