Souvenir shops can be found in Kingscote and Penneshaw. You can buy your supplies at supermarkets in those to centres too. Expect to pay more at small general stores selling food and drinks around the island. If you are going to camp in the remote parts of the island it is better to buy ahead since shops are rare and expensive usually limited to camp sites and resorts selling a few things especially in the west of the island.
As well as the ubiquitous wineries, there are a couple of produce places around the island.
Honey
At clifford's honey farm. they have an interesting sparkling honey drink called fresco. go into their back room and see a hive and a dvd showing how honey is harvested. stay in the shop area and buy lots of honey products and souvenirs. for 20c you can have a piece of chocolate covered honeycomb, which if you like chocolate covered honeycomb, may just be the highlight of your visit to the island. for 40c, you can have 2 pieces, and for just $2 you can have 10 pieces. it just doesn't get any better. try the honey ice-cream as well. it doesn't taste like honey, but it is really creamy and sweet.
Eucalyptus Oil
At emu ridge distillery. you can visit the shop - selling a wide range of eucalyptus products. do they really produce the lollies on site, or have they gone to the factory on the mainland and been shipped back to the shop? who knows. they have a wallaby and a kangaroo which jump around in the shop, although the shop owner insists they are free to come and go as they please. they also sell feral cat skins for a few dollars, hunted locally. it is an environmentally friendly, albeit diminutive answer to the lion skin in front of the fireplace.